●  Only enforceable promises in an agreement can be relied upon before the courts.

●  To be enforceable, a formation promise (promise made as part of an agreement where there is no existing agreement between these parties covering this subject matter) requires (a) an intention to be legally bound by the promise (known as intention to create legal relations), and (b) either the promise to be expressed in the form of a deed (indicating that any promise is taken seriously), or the party seeking to enforce the promise of the other to show they have given something in exchange for that promise (i.e. it is a bargain, as opposed to a gratuitous promise or gift).

●  Intention to be legally bound is judged objectively through the use of two presumptions. These may be rebutted by clear evidence to the contrary. Domestic or social agreements are presumed not to be intended to have legally enforceable consequences and to rebut this presumption there needs to be reliance, certainty of terms, and evidence of the seriousness of the promise. In contrast, parties to commercial agreements are presumed to intend to be legally bound unless there are clear words indicating the absence of a promise or that the parties have agreed to be bound in honour only.

●  Consideration means an act or a promise given in exchange for the promise (i.e. the price for which the other’s promise was bought). Consideration need not be adequate but must be sufficient. This means that the courts will not examine whether what has been given in exchange is of equivalent value but some acts or promises are not recognized by the law as being valid consideration, e.g. past consideration and the performance of an existing legal duty.

●  In order to enforce an alteration promise, a party must also show that it has provided consideration for, or purchased, that promise. Performing an existing contractual formation obligation was not recognized as sufficient consideration since there was no new exchange. But where there is an alteration promise, and that promise has been freely made (see the doctrine of duress), if the promisor receives a factual benefit from making the alteration promise, that promise might be treated as supported by consideration and enforceable.

●  In the context of alteration promises, the Court of Appeal (CA) has suggested that the promisor’s receipt of a factual or practical benefit can amount to consideration to support a promise to pay more money (Williams v Roffey Bros) and a promise to accept less money (MWB Business Exchange Centres Ltd v Rock Advertising Ltd), although this area of law is without clarity of principle.

●  Where an alteration promise is not supported by consideration, that promise might have some binding effect (although not the same binding effect as where consideration is present) through the doctrine of promissory estoppel. Promissory estoppel prevents a promisor from going back on a promise where to do so would be inequitable because the promisee has relied on that promise and the promise was freely given (again the doctrine of duress might be relevant).

●  There are some difficulties with the scope of promissory estoppel in English law.

●  As noted, in practice the enforceability of a promise may be determined by whether that promise was freely given since the doctrine of duress (coercion or threat to the person or property, or illegitimate pressure or threats to financial interests) renders any such promise voidable (and so liable to be treated as having no effect).