Mistake of Fact in UK Law

First, there must have been a mistake as to an existing fact, including a mistake as to the availability of evidence on a particular matter. Secondly, the fact or evidence must have been "established", in the sense that it was uncontentious and objectively verifiable. Fourthly, the mistake must have played a material (not necessarily decisive) part in the Tribunal's reasoning.

At present I do not wish to state the principle any less broadly than this: that the defence is available to a person whose position has so changed that it would be inequitable in all the circumstances to require him to make restitution, or alternatively to make restitution in full.

It is clear that any civilised system of law is bound to provide remedies for cases of what has been called unjust enrichment or unjust benefit, that is to prevent a man from retaining the money of or some benefit derived from another which it is against conscience that he should keep.

Corresponding to mistake as to the existence of the subject matter is mistake as to title in cases where unknown to the parties the buyer is already the owner of that which the seller purports to sell to him. To such a case Lord Westbury applied the principle that if parties contract under a mutual mistake and misapprehension as to their relative and respective rights the result is that the agreement is liable to be set aside as having proceeded upon a common mistake.

In other words the jury should be directed first of all that the prosecution have the burden or duty of proving the unlawfulness of the defendant's actions; secondly, if the defendant may have been labouring under a mistake as to the facts, he must be judged according to his mistaken view of the facts; thirdly, that is so whether the mistake was, on an objective view, a reasonable mistake or not.

On an appeal the court must decide any question of law arising from an award on the basis of a full and unqualified acceptance of the findings of fact of the arbitrators. It is irrelevant whether the court considers those findings of fact to be right or wrong. It also does not matter how obvious a mistake by the arbitrators on issues of fact might be, or what the scale of the financial consequences of the mistake of fact might be.

(i) there must be a common assumption as to the existence of a state of affairs; (ii) there must be no warranty by either party that that state of affairs exists; (iii) the non-existence of the state of affairs must not be attributable to the fault of either party; (iv) the non-existence of the state of affairs must render performance of the contract impossible; (v) the state of affairs may be the existence, or a vital attribute, of the consideration to be provided or circumstances which must subsist if performance of the contractual adventure is to be possible.