Unilateral Mistake in UK Law

(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.

So the agreement of A and B to purchase a specific article is void if in fact the article had perished before the date of sale. 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 the latter case it is necessary to show not only that each party to the contract had the same actual intention with regard to the relevant matter, but also that there was an “outward expression of accord” – meaning that, as a result of communication between them, the parties understood each other to share that intention.

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.

But as the alleged common intention ex hypothesis contradicts the written instrument, convincing proof is required in order to counteract the cogent evidence of the parties' intention displayed by the instrument itself.

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.

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.