This term appears to come from the Old French estoupail (or variation), which meant "stopper plug", referring to placing a halt on the imbalance of the situation. The term is related to the verb "estop" which comes from the Old French term estopper, meaning "stop up, impede."
- 1 Overview
- 2 Reliance-based estoppels
- 3 Indian Law
- 4 Other estoppels
- 5 See also
- 6 Notes
- 7 References
- 8 External links
OverviewEstoppel is essentially a rule of evidence whereby a person is barred from denying the truth of a fact that has already been settled. Where a court finds that a party has done something warranting a form of estoppel, that party is said to be "estopped" from making certain related arguments or claiming certain related rights. The defendant is said to be "estopped" from presenting the related defence, or the plaintiff is said to be "estopped" from making the related argument against the defendant. Lord Coke stated, "It is called an estoppel or conclusion, because a man's own act or acceptance stoppeth or closeth up his mouth to allege or plead the truth." The plea of estoppel is closely connected with the plea of waiver, the object of both being to ensure bona fides in day to day transactions.
Because estoppel is so factually dependent, it is perhaps best understood by considering specific examples such as the following:
- Example 1: A city entered into a contract with another party. The contract stated that it had been reviewed by the city's counsel and that the contract was proper. Estoppel applied to estop the city from claiming the contract was invalid.
- Example 2: A creditor unofficially informs a debtor that the creditor forgives the debt between them. Even if such forgiveness is not formally documented, the creditor may be estopped from changing its mind and seeking to collect the debt, because that change would be unfair.
- Example 3: A landlord informs a tenant that rent has been reduced, for example, because there was construction or a lapse in utility services. If the tenant relies on this statement in choosing to remain in the premises, the landlord could be estopped from collecting the full rent.
Promissory estoppel is often applied where there is an agreement without a consideration, or the consideration is future based; as a promise. When applied in defense by a defendant it may be called a 'shield', and where applied by a plaintiff it may be called a 'sword'. It is most commonly used as a 'shield', with some commentators stating that it can only be used as a shield, although this varies with jurisdictions.
Major typesThe main species of estoppel under English, Australian, and American laws are:
- Reliance-based estoppels: These involve one party relying on
something the other party has done or said. The party who
performed/spoke is the one who is estopped. Under English law, this
class includes estoppel by representation of fact, promissory estoppel
and proprietary estoppel (see Halsbury's Laws of England, Vol 16(2),
2003). Although some authorities have used language to suggest
reliance-based estoppels are mere rules of evidence, they are rules of substantive law.
- Estoppel by representation of fact (English law name), equitable estoppel (American law)
- Equitable estoppel (in English law), including
- Proprietary estoppel
- Promissory estoppel
- Estoppel by record: This frequently arises as issue/cause of action estoppel or judicial estoppel where the orders or judgments made in previous legal proceedings prevent the parties from relitigating the same issues or causes of action,
- Estoppel by deed (often regarded as technical or formal estoppels)—Where rules of evidence prevent a litigant from denying the truth of what was said or done
- Estoppel by silence or acquiescence: Estoppel that prevents a person from asserting something when he had the right and opportunity to do so earlier, and such silence put another person at a disadvantage.
- Laches: estoppel in equity by delay. Laches has been considered both a reliance-based estoppel, and a sui generis estoppel.
Reliance-based estoppelsReliance-based estoppels (at English law) include:
- by representation of fact, where one person asserts the truth of a set of facts to another;
- promissory estoppel, where one person makes a promise to another, but there is no enforceable contract; and
- proprietary estoppel, where the parties are litigating the title to land.
All reliance-based estoppels require the victimised party to show both inducement and detrimental reliance, i.e.:
- there must be evidence to show that the representor actually intended the victim to act on the representation or promise, or
- the victim must satisfy the court that it was reasonable for him or her to act on the relevant representation or promise, and
- what the victim did must either have been reasonable, or
- the victim did what the representor intended, and
- the victim would suffer a loss or detriment if the representor was allowed to deny what was said or done — detriment is measured at the time when the representor proposes to deny the representation or withdraw the promise, not at the time when either was made, and
- in all the circumstances, the behavior of the representor is such that it would be "unconscionable" to allow him or her to resile.
- there was a promise
- that was reasonably relied upon
- resulting legal detriment to the promisee
- justice requires enforcement of the promise
The test for unconscionability in the English and Australian courts takes many factors into account, including the behavior, state of mind and circumstances of the parties. Generally, the following eight factors are determinative:
- how the promise/representation and reliance upon it were induced;
- the content of the promise/representation;
- the relative knowledge of the parties;
- the parties' relative interest in the relevant activities in reliance;
- the nature and context of the parties' relationship;
- the parties' relative strength of position;
- the history of the parties' relationship; and
- the steps, if any, taken by the promisor/representor to ensure he has not caused preventable harm.
the ingredients for a proprietary estoppel should include, in principle, a proprietary claim made by a claimant and an answer to that claim based on some fact, or point of mixed fact and law, which the person against whom the claim was made could be estopped from asserting. To treat a “proprietary estoppel equity” as requiring simply unconscionable behaviour was a recipe for confusion. The remedy to which, on the facts as found by the judge, the claimant was entitled could be described neither as based on an estoppel nor as proprietary in character. His Lordship’s present view was that proprietary estoppel could not be prayed in aid to render enforceable an agreement declared by statute (s. 2 of the Law Reform (Miscellaneous Provisions) Act 1989) to be void. A claim for the imposition of a constructive trust to provide a remedy for a disappointed expectation engendered by a representation made in the course of incomplete contractual negotiations was misconceived and could not be sustained by reliance on unconscionable behaviour. The claimant was, however, entitled to a quantum meruit payment for his services in obtaining the planning permission.
Estoppel by representation of fact (Law of England and Wales)In English law, estoppel by representation of fact is a term coined by Spencer Bower. This species of estoppel is also referred to as "common law estoppel by representation" in Halsbury's Laws of England, vol 16(2), 2003 reissue.
Spencer Bower defines estoppel by representation of fact as follows:
Where one person (‘the representor’) has made a representation of fact to another person (‘the representee’) in words or by acts or conduct, or (being under a duty to the representee to speak or act) by silence or inaction, with the intention (actual or presumptive) and with the result of inducing the representee on the faith of such representation to alter his position to his detriment, the representor, in any litigation which may afterwards take place between him and the representee, is estopped, as against the representee, from making, or attempting to establish by evidence, any averment substantially at variance with his former representation, if the representee at the proper time, and in proper manner, objects thereto.A second definition comes from Sean Wilken and Theresa Villiers::para. 9.02
An estoppel by representation [of fact] will arise between A and B if the following elements are made out. First, A makes a false representation of fact to B or to a group of which B was a member. [It is not necessary to demonstrate A knew that the representation was untrue.] Second, in making the representation, A intended or [in the alternatively,] knew that it was likely to be acted upon. Third, B, believing the representation, acts to its detriment in reliance on the representation. [It must have been reasonable to rely on the representation.] Fourth, A subsequently seeks to deny the truth of the representation. Fifth, no defence to the estoppel can be raised by A.A representation can be made by words or conduct. Although the representation must be clear and unambiguous, a representation can be inferred from silence where there is a duty to speak or from negligence where a duty of care has arisen. Under English law, estoppel by representation of fact usually acts as a defence, though it may act in support of a cause of action or counterclaim.
Although there is some debate as to whether "unconscionability" is an element that English courts need to take into account when considering estoppel by representation of fact, the Australian courts clearly do.:para. 9-03
Equitable estoppel (American law)American equitable estoppel is the counterpart to estoppel by representation, and its elements are summarized as:
- facts misrepresented or concealed
- knowledge of true facts
- fraudulent intent
- inducement and reliance
- injury to complainant
- clear, concise, unequivocal proof of actus (not by implication)
Equitable estoppel (English law)Under English and Australian legal systems, estoppels in equity include promissory and proprietary estoppels. (Contrast with estoppel by representation, which is a claim (under the English system) at law.) For more information, see Promissory estoppel and Proprietary estoppel below.
The status of estoppel by representation of fact is less clear in Australia. Two seminal decisions purport to fuse common law and equitable estoppels into a single unified doctrine, but the New South Wales Court of Appeal continues to treat estoppel by representation at common law as distinct from equitable estoppel. This can be significant in deciding which court has jurisdiction to adjudicate on the issue.
Proprietary estoppelIn English law, proprietary estoppel is distinct from promissory estoppel. Proprietary Estoppel is not a concept in American law, but a similar result is often reached under the general doctrine of promissory estoppel.
Traditionally, proprietary estoppel arose in relation to rights to use the land of the owner, and possibly in connection with disputed transfers of ownership. Although proprietary estoppel was only traditionally available in disputes affecting title to real property, it has now gained limited acceptance in other areas of law. Proprietary estoppel is closely related to the doctrine of constructive trust.
J. Fry summarized the five elements for proprietary estoppel as:
- the claimant
- made a mistake as to his legal rights (typically because the actual owner attempted to convey the property, but the transfer is invalid or ineffective for some reason);
- did some act of reliance;
- the defendant
- knows of the existence of a legal right which he (the defendant) possesses, and which is inconsistent with the right claimed by the claimant;
- knows of the claimant's mistaken belief; and
- encouraged the claimant in his act of reliance.
Promissory estoppelThe doctrine of promissory estoppel prevents one party from withdrawing a promise made to a second party if the latter has reasonably relied on that promise.
English lawIn English law, a promise made without consideration is generally not enforceable. It is known as a bare or gratuitous promise. Thus, if a car salesman promises not to sell a car over the weekend, but does so, the promise cannot be enforced. But should the car salesman accept even one penny in consideration for the promise, the promise will be binding and enforceable in court. Estoppel is not an exception to this rule.
The doctrine of promissory estoppel was first developed in Hughes v. Metropolitan Railway Co  but was lost for some time until it was resurrected by Lord Denning in the controversial case of Central London Property Trust Ltd v. High Trees House Ltd.
Promissory estoppel requires:
- an unequivocal promise by words or conduct
- evidence that there is a change in position of the promisee as a result of the promise (reliance but not necessarily to their detriment)
- inequity if the promisor were to go back on the promise
Estoppel is an equitable (as opposed to common law) construct and its application is therefore discretionary. In the case of D & C Builders v. Rees the courts refused to recognise a promise to accept a part payment of £300 on a debt of £482 on the basis that it was extracted by duress. In Combe v. Combe Denning elaborated on the equitable nature of estoppel by refusing to allow its use as a "sword" by an ex-wife to extract funds from the destitute husband.
The general rule is that when one party agrees to accept a lesser sum in full payment of a debt, the debtor has given no consideration, and so the creditor is still entitled to claim the debt in its entirety. This is not the case if the debtor offers payment at an earlier date than was previously agreed, because the benefit to the creditor of receiving payment early can be thought of as consideration for the promise to waive the rest of the debt. This is the rule formulated in Pinnel's Case, and affirmed in Foakes v. Beer.
The decision of the Court of Appeal in Collier v P & MJ Wright (Holdings) Ltd suggests that the doctrine of promissory estoppel can now operate to mitigate the harshness of this common law rule. Moreover, Arden LJ held that allowing a creditor to renege on his promise to forebear seeking the balance of a debt in return for part payment would be, in and of itself, inequitable. Therefore, the only reliance that the promisee must demonstrate is the actual making of the part payment. This approach has been criticised as doing violence to the principle set down in Hughes and the extent to which the other members of the Court, namely Longmore LJ, agreed with it is uncertain.
Australian lawThe doctrine of promissory estoppel was adopted into Australian law in Legione v. Hateley; however, the plaintiffs were unsuccessful in that case because the reliance was unreasonable and the promise not unequivocal.
In fact, now Australian law has gone beyond the position espoused in the High Trees case;[further explanation needed] it has been extended successfully to cases where there is no pre-existing legal relationship between the two parties, and promissory estoppel can be wielded as a "sword", not just as a "shield". Mason CJ and Wilson J in Waltons Stores (Interstate) Ltd v Maher held that if estoppel is proven, it gives rise to an equity in favour of the plaintiff, and the court will do the minimum equity that is just in the circumstances. From this case, it is also possible for the promise to come from silence or inaction.
Stated by Brennan J in Waltons Stores:
To establish an equitable estoppel, it is necessary for the plaintiff to prove that 1) the plaintiff assumed that a particular legal relationship would exist between them (and in the latter case) that the defendant would not be free to withdraw from that expected legal relationship; 2) the defendant has induced the plaintiff to adopt that assumption or expectation; 3) the plaintiff acts or abstains from acting in reliance on the assumption or expectation; 4) the defendant knew or intended him to do so; 5) the plaintiff's action or inaction will occasion detriment if the assumption of expectation is not fulfilled; and 6) the defendant has failed to act to avoid that detriment whether by fulfilling the assumption or expectation of otherwise.As noted above, in Australian law, there is an element of unconscionability, which is satisfied if one party encourages the other party to create assumptions that lead to reliance. Today, the principle of estoppel may give birth to an enforceable obligation even without a consideration under the following conditions:
- dishonest behaviour of the promittant
- special relationship between the promittant and the beneficior (e.g.: duty of information)
- irreversible changement of the situation of the beneficior of the promise
American lawIn the many jurisdictions of the United States, promissory estoppel is generally an alternative to consideration as a basis for enforcing a promise. It is also sometimes referred to as detrimental reliance.
The American Law Institute in 1932 included the principle of estoppel into § 90 of the Restatement of Contracts, stating:
A promise which the promisor should reasonably expect to induce action or forbearance of a definite and substantial character on the part of the promisee and which does induce such action or forbearance is binding if injustice can be avoided only by enforcement of the promise.The distinction between promissory estoppel and equitable estoppel should be noted:
—Restatement (Second) removed the requirement that the detriment be "substantial".
Equitable estoppel is distinct from promissory estoppel. Promissory estoppel involves a clear and definite promise, while equitable estoppel involves only representations and inducements. The representations at issue in promissory estoppel go to future intent, while equitable estoppel involves statement of past or present fact. It is also said that equitable estoppel lies in tort, while promissory estoppel lies in contract. The major distinction between equitable estoppel and promissory estoppel is that the former is available only as a defense, while promissory estoppel can be used as the basis of a cause of action for damages.Suppose that B goes to a store and sees a sign that the price of a radio is $10. B tells the shopkeeper that he will get the money and come back later that day to purchase it; there is no discussion of price. The shopkeeper says that when B returns, he will be happy to deal with B as he deals with all his customers but that, if he sells all the radios (he has three), he will not be able to help B. Hearing this, B goes and sells his watch for $10 (it was really worth $15, but since B wanted the money right away, he chose not to wait for the best price). When B returns, the sign says $11, and the owner tells B that he has raised the price. In Equity, can you argue that the shopkeeper is estopped by conduct? B relied upon the implied representation that a radio would be sold for $10 when he returned with the money; B has sold his watch at a price lower than the market price, and thus he has acted to his detriment. (Note that if B's watch was worth $10, and he received a fair price, there would be no detriment.) But the problem is that the shopkeeper did not guarantee to hold one of the radios against the possibility of B's return nor did they agree a fixed price. The shopkeeper's conscience might have been affected if he had known that B was going home to collect the money and would definitely return to buy one of the three radios. Indeed, in some common law jurisdictions, a promise by the shopkeeper to hold a specific radio would create a binding contract, even if B had to go for the money. A promise to pay the owner in the future is good consideration if it is made in exchange for a promise to sell a specific radio (one from three is probably sufficiently specific): one promise in exchange for a second promise creates equal value. So the shopkeeper's actual words and knowledge are critical to deciding whether either a contract or an estoppel arises.
—28 Am Jur 2d Estoppel and Waiver § 34
For an example of promissory estoppel in the construction industry, suppose that B Ltd consolidates estimates from a number of subcontractors and quotes a single price on a competitive tender. The client accepts B Ltd's quote and construction begins. But one of the subcontractors then claims reimbursement above its original estimate and, because of this change, B Ltd cannot profit from the works. If both parties knew that the accuracy of the individual estimates was critical to the success of the tender and the profitability of the contract as a whole, a court might apply promissory estoppel and allow B Ltd to pay only what the subcontractor originally estimated rather than the new, higher price. But, if both parties hoped that there would be an opportunity to increase the contract prices to reflect additional expenditure, the subcontractor's conscience would not be as limited in seeking a higher payment and B Ltd might be penalised for not building an adequate contingency sum into the tendered price.
One contentious point during the drafting of the Restatement was how to calculate the amount of damages flowing from a promissory estoppel. During the deliberations, the following example was considered: a young man's uncle promises to give him $1,000 to buy a car. The young man buys a car for $500, but the uncle refuses to pay any money. One view was that the young man should be entitled to $1,000 (the amount promised), but many believed that the young man should only be entitled to $500 (the amount he actually lost). The language eventually adopted for the Second Restatement reads: "The remedy granted for breach may be limited as justice requires." — a formula which leaves quantification to the discretion of the court.
Indian LawSection 115 in the Indian Evidence Act defines Estoppel, to mean.- When one person has, by his declaration, act or omission, intentionally caused or permitted another person to believe a thing to be true and to act upon such belief, neither he nor his representative shall be allowed, in any suit or proceeding between himself and such person or his representative, to deny the truth of that thing.
Illustration A intentionally and falsely leads B to believe that certain land belongs to A, and thereby induces B to buy and pay for it. The land afterwards becomes the property of A, and A seeks to set aside the sale on the ground that, at the time of the sale, he had no title. He must not be allowed to prove his want of title.
No estoppel against constitution or fundamental rightsThere can be no estoppel against the Constitution of India. The Constitution is not only the paramount law of the land, but is the source and substance of all laws. Its provisions are conceived in public interest and are intended to serve a public purpose. The doctrine of estoppel is based on the principle that consistency in word and action imparts certainty and honesty to human affairs. If a person makes a representation to another, on the faith of which the latter acts, to his prejudice, the former cannot resile from the representation made by him. He must make it good. This principle can have no application to representations made regarding the assertion or enforcement of fundamental rights. Fundamental rights are undoubtedly conferred by the Constitution upon individuals which have to be asserted and enforced by them, if those rights are violated. But, the high purpose which the Constitution seeks to achieve by conferment of fundamental rights is not only to benefit individuals but to secure the larger Interests of the community. No individual can barter away the freedoms conferred upon him by the Constitution. A concession made by him in a proceeding, whether under a mistake of law or otherwise, that he does not possess or will not enforce any particular fundamental right, cannot create an estoppel against him in that or any subsequent proceeding. Such a concession, if enforced, would defeat the purpose of the Constitution.
PaisEstoppel in pais (literally “by act of notoriety", or "solemn formal act”) is the historical root of common law estoppel by representation and equitable estoppel. The terms Estoppel in pais and equitable estoppel are used interchangeably in American law.
ConventionEstoppel by convention in English law (also known as estoppel by agreement) occurs where two parties negotiate or operate a contract but make a mistake. If they share an assumption, belief or understanding of how the contract will be interpreted or what the legal effect will be, they are bound by that belief, assumption or understanding if:
- (i) they both knew the other had the same belief, and
- (ii) they both based their subsequent dealings on those beliefs.
Estoppel by convention is most commonly invoked if one party wishes to rely on pre-contract negotiation as an aid to construction of the contract.
AcquiescenceEstoppel by acquiescence may arise when one person gives a legal warning to another based on some clearly asserted facts or legal principle, and the other does not respond within "a reasonable period of time". By acquiescing, the other person is generally considered to have lost the legal right to assert the contrary.
As an example, suppose that Jill has been storing her car on Jack's land with no contract between them. Jack sends a registered letter to Jill's legal address, stating: "I am no longer willing to allow your car to stay here for free. Please come get your car, or make arrangements to pay me rent for storing it. If you do not do so, within 30 days, I will consider the car abandoned and will claim ownership of it. If you need more time to make arrangements, please contact me within 30 days, and we can work something out." If Jill does not respond, she may be said to have relinquished her ownership of the car, and estoppel by acquiescence may prevent any court from invalidating Jack's actions of registering the car in his name and using it as his own.
DeedEstoppel by deed is a rule of evidence arising from the status of a contract signed under seal — such agreements, called deeds, are more strictly enforced than ordinary contracts and the parties are expected to take greater care to verify the contents before signing them. Hence, once signed, all statements of fact (usually found in the opening recital which sets out the reason(s) for making the deed) are conclusive evidence against the parties who are estopped from asserting otherwise.
Conflict estoppel"[An] inconsistent position, attitude or course of conduct may not be adopted to loss or injury of another". For example, as between two or more claimants, a party that takes multiple and inconsistent legal positions is estopped to assert its positions against another consistent and certain claim, i.e. preferential treatment for certain over uncertain claims.
Issue estoppelIssue estoppel (more commonly known as issue preclusion) prevents, in some cases, an issue that has already been litigated and decided on the merits from being re-litigated, even when the parties are different. In the world of crime, some cases have achieved notoriety, e.g. in the Birmingham Six saga, the House of Lords ruled in Hunter v. Chief Constable of the West Midlands Police (1982) that issue estoppel applied. Lord Diplock said:
[This case] concerns the inherent power which any court of justice must possess to prevent misuse of its procedure in a way which, although not inconsistent with the literal application of its procedural rules, would nevertheless be manifestly unfair to a party to litigation before it, or would otherwise bring the administration of justice into disrepute among right-thinking people.
- Acquiescence and Estoppel by acquiescence
- Assignor estoppel
- Collateral estoppel (USA)
- De facto corporation and corporation by estoppel (USA)
- Direct estoppel
- Estoppel by deed (USA)
- Judicial estoppel
- Licensee estoppel
- Principle of venire contra factum proprium non valet in legal systems based on civil law
- Prosecution history estoppel also known as file-wrapper estoppel (USA)
- 28 Am Jur 2d Estoppel and Waiver § 1
- 2 Coke, Littleton 352a
- India -- Olga Tellis v. Bombay Municipal Corporation (1985.07.10) (Right to Life and Livelihood for Homeless)
- Speckman v. City of Indianapolis, 540 N.E.2d 1189, 1191 (Ind. 1989).
- Halsbury's Laws of England, Vol 16(2), 2003
- Spence, Michael (1999). Protecting Reliance: The Emergent Doctrine of Equitable Estoppel. Hart Publishing. pp. 60–66. ISBN 9781901362626.
- Cobbe v Yeoman's Row  UKHL 55
- Bower, Spencer (2004). The Law relating to Estoppel by Representation (4th ed.). para. I.2.2.
- Wilken, Sean; Villiers, Theresa (2002). The Law of Waiver, Variation and Estoppel (2nd ed.). Oxford University Press.
- The Commonwealth v Verwayen (1990) 170 CLR 394 at 444 per Deane J.
- Cox, Lucian B. (1951). Equity: Principles and procedures in Virginia and West Virginia. § 288.
- Aspex Eyewear v. Clariti Eyewear Court Decision
- Joel Leeman, "Silence is Deadly: The Peril of Inaction After Calling Someone an Infringer", Sunstein IP Update (June 2010)
- Waltons Stores (Interstate) v Maher (1988) 164 CLR 387 and Commonwealth v Verwayen (1990) 170 CLR 394
- Byron Shire Council v Vaughan  NSWCA 158
- See Meagher, Gummow & Lehane's Equity: Doctrines & Remedies, 4th edition, Butterworth: 2002, Chapter 17 and Pakinson, The Principles of Equity, 2nd edition, LBC: 2003, Chapter 7.
- Wilmott v Barber (1880) 15 Ch D 96
- Dillwyn v Llewelyn (1862) 4 De G.F.& J. 517 C.A. See also Inwards v Baker  2 Q.B. 29, C.A.
- Central London Property Trust Ltd v. High Trees House Ltd  K.B. 130
- Combe v. Combe (1951) 2 KB 215
- Pinnel's Case (1602) 5 Co Rep 117a
- Foakes v. Beer (1884) 9 App Cas 605
- Collier v P & MJ Wright (Holdings) Ltd 2008 1 WLR 643
- Legione v. Hateley (1983) 152 CLR 406
- Waltons Stores (Interstate) Ltd v Maher (1988) 164 CLR 387
- Chartbrook Ltd and another v Persimmon Homes Ltd and another  UKHL 38
- Brand v. Farmer’s Mut. Protective Assoc of Texas, Tex. App 95 S.W.2d 994, 997
- Judgments - Polanski (Appellant) v. Conde Nast Publications Limited (Respondents) UK Parliament publications > Business > Lords Publications > Judgment Index > Judgment. Paragraph 86
- On issue estoppel: House of Lords - Arthur JS Hall and Co. v. Simons
- Convergence of estoppels (1) : High Court of Australia - Walton Stores (Interstate) Ltd v Maher
- Convergence of estoppels (2) : High Court of Australia - The Commonwealth v Verwayen
- Review of the state of convergence: High Court of Australia - Giumelli v Giumelli
- The 'Lectric Law Library - Estoppel Defined and Explained
- David Swarbrick. Estoppel (England) - 1980- 1984, website of swarb.co.uk "Estoppel, in its various forms including proprietary and equitable estoppels".
- Punishment and Proportionality: The Estoppel Approach, by Stephan Kinsella, a rights theory based on the concept of estoppel