'Lost' was a British special forces operation by the French 2^nd Régiment de Chasseurs Parachutistes (4^th Special Air Service) to secure intelligence in the Serent area of Brittany in German-occupied France (23 June/18 July 1944).

Under the command of Major Oswald A. J. Carey-Elwes, the eight-man party was parachuted into Brittany to establish links, on 30 June, with the 'Dingson' party. The two parties then operated alongside each other, undertaking offensive patrols and arming the local resistance forces. The operation resulted in the killing or wounding of 2,000 or more Germans, and the success of the operation as a whole is attested by the fact that the German diverted major units from other parts of France in an effort to contain the operations of the SAS and the French resistance forces in the region of Brittany.