In England, Wales and Northern Ireland, where a party to a contract lacks mental capacity the contract may be unenforceable if it can be shown that a firm knew – or should reasonably have known – that the customer lacked capacity at the time he/she entered into the contract.

In Scotland, if it can be shown that the customer lacked the capacity to enter into an informed contract (regardless of what the firm knew or ought to have known) then the contract could be declared unenforceable.