An estoppel prevents an individual, or a party to court proceedings, from denying a state of affairs which already exists.

It works mainly in two quite separate contexts.

The first is a matter of evidence. Where a court has already decided an issue or issues between two parties, one of them (A) may not argue that issue (issue estoppel) or the whole case (cause of action estoppel) all over again (also called res judicata: the thing or case has been adjudicated upon). The judge has decided the point. A is estopped from arguing the issue (part of the case), or the whole case, all over again before another judge. It brings that particular piece of litigation to an end and stops a person being troubled all over again by the same case (see Virgin Atlantic Airways Ltd v Zodiac Seats UK Ltd [2013] UKSC 46, [2014] AC 160 for a recent definition).

Proprietary estoppel applies where one person (A) has made promise of land to another (B); and in reliance on that promise B has performed his side of a bargain (eg to work on A’s farm for less or no wages, or incurred some other detriment). If A tries to get out of his side of the deal, but B has carried out his side of the bargain, an estoppel will hold A to his side of the deal. This operates commonly in family farming cases (see eg Davies v Davies [2016] EWCA Civ 463, [2017] 1 FLR 1286 or Habberfield v Habberfield [2019] EWCA Civ 890).

Promissory estoppel applies similar principles where the promise is other than for land, and prevents the promissor A getting out of his deal with B who has carried out his side of a bargain.