Estoppel in English law is a doctrine that may be used in certain situations to prevent a person from relying upon certain rights, or upon a set of facts (e.g. words said or actions performed) which is different from an earlier set of facts.

Estoppel could arise in a situation where a creditor informs a debtor that a debt is forgiven, but then later insists upon repayment. In a case such as this, the creditor may be estopped from relying on their legal right to repayment, as the creditor has represented that he no longer treats the debt as extant. A landlord may tell his tenant that he is not required to pay rent for a period of time ("you don't need to pay rent until the war is over"). Until the war is over, the landlord would be "estopped" from claiming rents during the war period. Estoppel is often important in insurance law, where some actions by the insurer or the agent estop the insurer from denying a claim.

There are a huge array of different types of estoppel which can arise under English law. It has been judicially noted on more than one occasions that the link between them is often 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 generally abandoned any attempt to create a single general underlying rationale or principle; in First National Bank plc v Thompson [1996] Ch 231 CA Lord Millett said: "the attempt... to demonstrate that all estoppels... are now subsumed in the single and all-embracing estoppel by representation and that they are all governed by the same principle [has] never won general acceptance."