Although a brilliant scholar and talented linguist, John Cairncross was best known as the "Fifth Man" and one of the Soviet Union's most important spies in Britain.
He was widely suspected of providing the information that started the Soviet nuclear programme. The quality of the information he provided meant that the Russians regarded him as a member of the notorious "ring of five" along with his fellow Cambridge graduates Guy Burgess, Donald Maclean, Kim Philby and Anthony Blunt.
But Cairncross railed against the description, saying he was a loner and always maintained that he never passed on information that was harmful to Britain.
He was born in 1913, the son of a Glaswegian shopkeeper and a primary school teacher. He joined the Communist Party while at Cambridge University in 1935 and was recruited the following year by the KGB after topping the entrance exams to the Foreign Office.
At the outbreak of the Second World War, he was posted to the Cabinet Office. At the time of the rapprochement between Germany and the Soviet Union, he passed on secrets to Guy Burgess.
He was then posted to the code-breaking centre at Bletchley Park, where he was so disgusted to learn that the Soviets had been kept in the dark about intelligence gained from breaking the Enigma ciphers that he passed over hundreds of unravelled German signals.
His role was uncovered in 1951 after the defections of Burgess and Maclean, but he did not make a full confession until 1964. The Government did not consider it worth prosecuting him. By then, he had long since been dismissed from the Treasury, without a pension, but his career prospects appeared not to have been harmed by his treachery. He was a distinguished scholar, known in France as an authority on Moliere. He also worked around the world for the United Nations.
His name came to light in 1979 after the Anthony Blunt confessions. His role was confirmed by the KGB double agent Oleg Gordievsky, who said he passed over "literally tons of documents". In 1991, Cairncross publicly admitted he had spied for the Russians. He died four years later, aged 82.Reuse content