Unilateral Mistake in UK Law

What is relevant in such a case is the subjective intention of the settlor. Although, as I have said, there is no legal requirement of an outward expression or objective communication of the settlor's intention in such a case, it will plainly be difficult as a matter of evidence to discharge the burden of proving that there was a mistake in the absence of an outward expression of intention.

(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.

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.

It does not matter if the mistake is due to carelessness on the part of the person making the voluntary disposition, unless the circumstances are such as to show that he deliberately ran the risk, or must be taken to have run the risk, of being wrong.

I would provisionally conclude that the true requirement is simply for there to be a causative mistake of sufficient gravity; and, as additional guidance to judges in finding and evaluating the facts of any particular case, that the test will normally be satisfied only when there is a mistake either as to the legal character or nature of a transaction, or as to some matter of fact or law which is basic to the transaction.

The general rule at common law is that if one party has made a mistake as to the terms of the contract and that mistake is known to the other party, then the contract is not binding. The reasoning is that although the parties appear, objectively, to have agreed terms, it is clear that they are not in agreement. Some of the cases talk of such a contract being “void”, but I think it is clearer to say that there was never a contract at all.

Whether there was in any particular case knowledge of the intention and mistake of the other party must be a question of fact to be decided upon the evidence. Basically it appears to us that it must be such as to involve the defendant in a degree of sharp practice. Basically it appears to us that it must be such as to involve the defendant in a degree of sharp practice.