Contract Mistakes in UK Law

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.

They can be supported on the ground that it is of paramount importance that contracts should be observed: and that if parties honestly comply with the essentials of the formation of contratcs, i.e., agree in the same terms on the same subject matter, they are bound: and must rely on the stipulations of the contract for protection from the effect of facts unknown to them.

"In the contemplation of both parties fundamental to the continued validity of the contract," "a foundation essential to its existence," "a fundamental reason for making it" are phrases found in the important judgment of Scrutton L.J. in the present case. Nothing is more dangerous than to allow oneself liberty to construct for the parties contracts which they have not in terms made by importing implications which would appear to make the contract more businesslike or more just.

Mistakes have relevance in the law of contract in a number of different circumstances. They may prevent the mutuality of agreement that is necessary for the formation of a contract. It may be that each party mistakenly believes that he has entered into such a contract in circumstances where an objective appraisal of the facts reveals that no agreement has been reached as to the terms of the contract.

(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.

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.

Where this leaves the common law doctrine of mistake, as it seems to me, is as follows. First, the doctrine is not based on an inquiry into the subjective beliefs of the parties but on an objective analysis of what they agreed. Second, the doctrine does not rest on the notion that the parties have impliedly agreed what is to happen in the event that an assumption underlying the contract proves to be false. It does, however, involve a question of construction of the contract.