Promissory Estoppel
An equitable doctrine preventing a party from going back on a clear and unequivocal promise to forego their strict legal rights, where the other party has relied on that promise and it would be inequitable to allow the promisor to resile from it. Established in Central London Property Trust v High Trees House [1947] by Denning J. It operates as a 'shield, not a sword' — it can only be used as a defence, not to found a cause of action (Combe v Combe [1951]).
Independent editorial summary — not the official statute text. Read the official version on legislation.gov.uk.
The Supreme Court's press summary in Rock Advertising Ltd v MWB Business Exchange Centres Ltd [2018] UKSC 24 discusses estoppel as the doctrine that guards against the injustice that can arise where a party acts on an informally varied contract. Addressing 'No Oral Modification' clauses, the Court observed that “the safeguard against injustice lies in the various doctrines of estoppel,” but set a demanding threshold for reliance on it.
That threshold requires more than an informal promise standing alone. The Court held that “reliance on an estoppel would require, at the very least: (i) some words or conduct unequivocally representing that the variation was valid notwithstanding its informality and (ii) for this purpose, something more than the informal promise itself.” This underscores that promissory (and related) estoppel doctrines operate as a safeguard against the strict enforcement of formal contractual requirements only where the party seeking to rely on it can point to something clear and unequivocal beyond the promise itself, consistent with the doctrine's origins in Central London Property Trust v High Trees House [1947] and its defensive, 'shield not sword' character confirmed in Combe v Combe [1951].
Related terms
Official sources
This explanation is drawn from the official sources below; every substantive statement is verified against them. For advice on a specific matter, see our find help page.