John Cairncross (25 July 1913 – 8 October 1995) was a British civil servant who became an intelligence officer and spy during the Second World War. As a Soviet double agent, he passed to the Soviet Union the raw Tunny decryptions that influenced the Battle of Kursk. He was alleged to be the fifth member of the Cambridge Five. He was also notable as a translator, literary scholar and writer of non-fiction.