What is the effect of mutual mistake in contract under English law?

Under English law, mutual mistake is a vitiating factor that can render a contract voidable. A mutual mistake occurs when both parties to a contract hold a common, but mistaken, belief about a material fact that forms part of the contract. If a contract is formed based on a mutual mistake, either party may seek to have the contract declared voidable. The effect of a voidable contract is that it remains valid and binding unless and until one of the parties seeks to have it set aside. If a contract is set aside due to mutual mistake, the parties are placed in the position they would have been in if the contract had never been formed. This typically involves a restitution of any benefits received under the contract, and a return to the position that existed prior to the contract being formed. In summary, the effect of mutual mistake in a contract under English law is that the contract may be voidable, meaning it remains valid and binding unless and until one of the parties seeks to have it set aside. If set aside, the parties are placed in the position they would have been in if the contract had never been formed.