'Ruthless' was a British unrealised deception operation to gain access to a German Enigma encryption machine (summer/autumn 194o).

The undertaking was planned by the Admiralty’s Operational Intelligence Centre, a primary beneficiary of 'Ultra' decrypts of German message traffic, and was developed by Commander Ian Fleming, personal assistant to Rear Admiral John Godfrey, the Director of Naval Intelligence at the OIC.

The plan was that the bomber, in German markings and flown by a five-man crew wearing German uniforms and covered with 'blood' and bandages, to follow on behind aircraft returning from a night bombing raid. As it passed over the middle of the English Channel, it would cut one engine and lose height with smoke pouring from a 'candle' in the tail, send an SOS distress signal and ditch into the sea. The crew would then take to a rubber dinghy, having ensured that the bomber sank before the Germans could identify it, and wait to be rescued by a German naval vessel, preferably a minesweeper. When on board, the 'survivors', one of whom was to speak fluent German, would then kill the German crew, and sail the captured vessel, complete with its Enigma machine and documentation, to a port on the south coast of England.

The plan was not attempted as, on the selected day, there were no German warships in the English Channel or, according to other sources, there were second thoughts about the plan’s basic viability.