John Robert Fisher Jeffreys (25 January 1916 – 13 January 1944) was a British mathematician and World War II codebreaker. A research fellow at Downing College, Cambridge, Jeffreys joined the codebreaking efforts at Bletchley Park in September 1939 alongside fellow Cambridge mathematicians Gordon Welchman, with whom he had previously worked closely, and Alan Turing. These three, together with Peter Twinn and working under Dilly Knox, formed the research section working on the German Enigma machine, and were housed in "The Cottage" at Bletchley Park. Gordon Welchman later recalled: