Estoppel

Estoppel is a judicial device whereby a court may prevent or "estop" a person from making assertions or from going back on their word. The person barred from doing so is said to be "estopped". Estoppel may prevent someone from bringing a particular claim. In common law legal systems, the legal doctrine of estoppel is based in both common law and equity.
Estoppel is also a concept in international law.

Types of estoppel

There are many different types of estoppel, but the common thread between them is that a person is restrained from asserting a particular position in law where it would be inequitable to do so. By way of illustration:

If a landlord promises a tenant that he will not exercise his right to terminate a lease, and relying upon that promise the tenant spends money improving the premises, the doctrine of promissory estoppel may prevent the landlord from exercising a right to terminate, even though his promise might not otherwise have been legally binding as a contract. The landlord is precluded from asserting a specific right.
If a person brings legal proceedings in one country claiming that he was injured by a second person negligently, and the courts of that country determine that there was no negligence, then under the doctrine of issue estoppel the first person cannot normally argue before the courts of another country that the second person was negligent. The first person is precluded from asserting a specific claim.

Estoppel is traditionally an equitable doctrine. Accordingly, it is sometimes said that any person wishing to assert an estoppel must come to the court with "clean hands".
The doctrine of estoppel is often confused with the doctrine of waiver. It also substantially overlaps with, but is distinct from, the equitable doctrine of laches.

Etymology and usage

"Estop" is a verb of Anglo-Norman origin meaning "to seal up", while the noun "estoppel" is based on Old French estoupail. When 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 estopped from presenting the related defense, or the plaintiff is 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."

Overview

Estoppel is sometimes said to be a rule of evidence whereby a person is barred from leading evidence of a fact that has already been settled or they are otherwise precluded from asserting, but that may be an oversimplification. Firstly, although some estoppels relate to preventing a party from asserting facts, others relate to preventing a party from asserting a right or a claim. Secondly, under the conflict of laws in common law jurisdictions, matters of evidence are usually treated as procedural matters for the law of the local court, whereas it is generally accepted that an estoppel may affect substantive rights and are therefore matters to be determined by the proper law that governs the particular issue.
There are many different types of estoppel that can arise under common law legal systems. It has been judicially noted on more than one occasion that the link between them is often somewhat tenuous. Treitel on Contracts notes that "unconscionability ... provides the link between them", but they nevertheless have "separate requirements and different terrains of application". The courts have long abandoned an attempt to create a single general underlying rationale or principle:
The plea of estoppel is often closely connected with the plea of waiver, the object of both being to ensure bona fides in day-to-day transactions. It is also related to the doctrines of variation and election. It is applied in many areas of contract law, including insurance, banking, and employment. In English law, the concept of legitimate expectation in the realm of administrative law and judicial review is estoppel's counterpart in public law.
Promissory estoppel is often applied where there is a promise or an agreement made without consideration. When used as a defense by a defendant it is sometimes called a "shield", and when used affirmatively by a plaintiff it is sometimes 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.

Examples

Estoppel can be understood by considering examples such as the following:

A city entered into a contract with another party. The contract stated that it had been reviewed by the city's legal counsel and that the contract was proper. Estoppel applied to estop the city from claiming the contract was invalid.
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.
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.
Types

Some types of estoppel under English, Australian, and American laws are as follows:

In civil cases

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. This category is discussed [|below].
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: Situations 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 after a litigant deliberately and avoidably delays an action so as to disadvantage an adversary.
In American criminal cases
Entrapment by estoppel: In American criminal law, although "ignorance of the law is no excuse" is a principle which generally holds for traditional crimes, courts sometimes allow this excuse as a defense, when defendant can show they reasonably relied on an interpretation of the law by the public official charged with enforcement or interpretation of that law, such as a regulatory official. The defense is based on "fairness" principles in the Fifth Amendment. It generally involves determination of fact, not of law, except as to determination of whether the interpreting government official had the legal status asserted by the defense.
In patent law

Estoppel is relevant and applied in U.S. patent law.
Acceptance of background documents by the patent office into a granted patent effectively gives rise to estoppel on any opposition based on the same documents. In effect, when the background section lists a document, it cannot be used to dispute the novelty of the patent. This is because the acceptance of the background by the patent office implies that the matter presented is already considered to be within the "state of the art" at the time when the patent is granted, and still distinct from the patent claim. For the patent applicant, sufficient inclusion of relevant prior art therefore protects the patent from dispute.
There are many types of estoppel applied in patent law, such as collateral estoppel; prosecution history estoppel ; and estoppel, which estops the petitioner from re-applying the same grounds in court that they already applied in an inter partes review.

Reliance-based estoppels

Reliance-based estoppels 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.

Both Halsbury's and Spencer Bower describe these three estoppels collectively as estoppels by representation. More simply, one party must say or do something and see the other party rely on what is said or done to change behavior.
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.

Simply put, promissory estoppel has four necessary elements which the plaintiff must prove:

there was a promise
that was reasonably relied upon
resulting in legal detriment to the promisee
justice requires enforcement of the promise

Estoppel by representation of fact and promissory estoppel are mutually exclusive: the former is based on a representation of existing fact, while the latter is based on a promise not to enforce some pre-existing right. A promissory estoppel operates only between parties who, at the time of the representation, were in an existing relationship, while this is not a requirement for estoppel by representation of fact.
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.

But in Cobbe v Yeoman's Row, Lord Scott of Foscote stated the following: