Non est factum
Latin for ‘not my deed’, non est factum is a common-law defence to an action on a deed or other signed instrument, open to a defendant who can prove his/her ignorance of the nature of the instrument at the time of execution or that the instrument was not executed by him/her.
Where the defence is successful (and this will never be the case where a defendant who actually signed is literate and has full legal capacity to contract), the effect is to avoid the instrument and the rights of those claiming under it, rendering the contract void ab initio (from the beginning).
Authoritative case law for non est factum can be found in the judgment of Petelin v Cullen, where a contract between Petelin (who was illiterate and spoke little English) and Cullen for an option to purchase land, was void ab initio due to a successful claim by Petelin of non est factum.