Non est factum (Latin for "it is not [my] deed") is a doctrine in contract law that allows a signing party to escape performance of the agreement. A claim of non est factum means that the signature on the contract was signed by mistake, without knowledge of its meaning, but was not done so negligently. A successful plea would make the contract void ab initio.
Non est factum is difficult to claim as it does not allow for negligence on the part of the signatory, i.e. failure to read a contract before signing it will not allow for non est factum. In a successful case, the fundamental basis of the signed contract must be completely different from what was intended. In Lloyds Bank v Waterhouse (1990) a father acted as a guarantor to his son's debt when purchasing a farm. The father was illiterate and signed the bank document under the belief that he was acting as the guarantor for the farm only, when the contract was actually for all the debt accumulated by the son. As he was illiterate, this was a mistake as to the document signed and the father was successful in claiming non est factum.
Another notable case on non est factum is Foster v Mackinnon (1869) LR 4 CP 704 where an elderly man signed a bill of exchange but was only shown the back of it. He was granted a new trial.