Can the doctrine of common law be used as a cause of action?

The doctrine can be used only as a defence, not as a cause of action. Denning L.J. said in Combe v. Combe, [1951] 2 K.B. 215, [1951] 1 All E.R. 767 at 769: “It does not create new causes of action where none existed before. It only prevents a party from insisting on his strict legal rights when it would be unjust to allow him to do so, having regard to the dealings which have taken place between the parties.”

However, that rule, in my view, is not an obstacle in the way of the plaintiff in the present case. He has not pleaded the estoppel in his statement of claim as a cause of action in itself. He has dealt with it in his reply with a view of meeting the defence that the release that he signed is an answer to his claim. As Lord Denning said in Combe v. Combe, “It may be part of a cause of action, but not a cause of action in itself” [p. 770].