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Non est factum meaning
What does Non est factum mean?
A narrow common law defence allowing a person to deny that a signed deed, contract, guarantee or mortgage is their act (“not my deed”) where, without negligence, they were fundamentally mistaken as to the nature and effect of what they signed. It is established by case law rather than statute; the leading modern authority in England and Wales (followed in Northern Ireland) is Saunders v Anglia Building Society [1971] AC 1004.
Key features: the document must be radically different from what the signatory believed; mere failure to read, misunderstanding of terms, or carelessness will defeat the plea. It commonly arises where a blind, illiterate or linguistically disadvantaged signer relies on a misleading explanation. It is distinct from forgery and undue influence.
The burden of proof lies on the party pleading it. If made out, the instrument is void ab initio against that party, and courts confine the doctrine strictly to protect transactional certainty and third-party rights.
Usage across the UK and Ireland is broadly consistent. In Scotland the analysis is framed through reduction for essential error, rather than the label non est factum, but the threshold and practical outcome are similarly stringent.
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View the related Practice Notes about Non est factum
PRACTICE NOTES
Mistake of Fact and Law in English Contract Law: Common, Mutual, Unilateral, Identity, Non Est Factum; Risk Allocation, Voidness, Rectification and Construction; Statutory Exceptions
This Practice Note explores the doctrine of mistake in contract law. It surveys common, mutual and unilateral mistake, errors as to identity, and mistake regarding the signed document (non est factum). It also considers the effect of each type on the contract and how mistakes can be addressed by rectification or by construction. For guidance on dealing with errors in the execution of documents, see Practice Note: Deeds—Failure to comply with formalities and other defects and our Execution collection, in particular, The Basics—Q&As—Mistakes in executing documents. For further help where parties choose to fix a mistake by agreeing an amendment to the operative parts of a contract, see Practice Note: Contract variation. What is a mistake? A mistake is a wrong belief held by one or both parties at the point of contract formation. A mistake may relate to the: subject matter or the terms of the contract identity of the counterparty nature of the transaction Do not confuse...
21 May
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PRACTICE NOTES
Void, voidable and unenforceable contracts: common and unilateral mistake, mistaken identity, non est factum, statutory avoidance, illegality, third-party rights, restitution and ultra vires
This Practice Note outlines when a contract may be treated as void and the consequences that follow. It differentiates void, voidable and unenforceable agreements, and explores voidness arising from: common mistake, a unilateral mistake as to contractual terms where the other party is aware, mistaken identity, non est factum, and statutory avoidance, while also recognising illegality as a distinct category. For a tabular overview of illustrative judgments on contract law since 1 January 2020, see Practice Notes: Contract disputes—illustrative decisions (2026) Contract disputes—illustrative decisions (2024–2025) [Archived] Contract disputes—key and illustrative decisions (2020–2023) [Archived] When is a contract a void contract? A void contract is one with no legal effect whatsoever. A contract will be void where: the parties have contracted under a fundamental common mistake one party agrees on mistaken terms and the other party knows of that mistake one party is mistaken as to the other party’s identity a party signs a document...
22 May
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