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.

The most significant aspect of his work was helping the Soviets defeat the Germans in battle during the Second World War; he may also have told Moscow that the US was developing an atomic bomb.

According to The Washington Post, the suggestion that John Cairncross was the "fifth man" of the Cambridge ring was not confirmed until 1990, by Soviet double-agent Oleg Gordievsky.

Cairncross grew up in Lesmahagow, a small town on the edge of moorland, near Lanark in the Central Belt of Scotland, and was educated at Lesmahagow Higher Grade School (where his name appears as the 1928 winner of the Dux prize); Hamilton Academy; the University of Glasgow; the Sorbonne; and Trinity College, Cambridge, where he studied French and German.

[8] An article in the Glasgow Herald on 29 September 1936 noted that Cairncross had scored an "outstanding double success of being placed 1st in the Home List and 1st in the competition for the Foreign Office and the Diplomatic Service" and that he had been placed fifth in the University of Glasgow bursary competition of 1930 and was also a Scholar and Bell Exhibitioner at Trinity College, Cambridge.

Sir Alec also recalled that John "was a prickly young man, who was difficult to argue with and resented things rather easily".

[11] It was while he was working with the Foreign Service (c. 1936) that he was recruited as a spy for the Soviets by James Klugmann of the Communist Party of Great Britain.

[16] Cairncross smuggled Tunny decrypts due to be destroyed out of Hut 3 in his trousers, transferring them to his bag at the railway station before going to meet his NKVD contact in London, Anatoli Gorsky.

[17] The raw transcripts decrypted by Colossus were passed to intelligence officers at Bletchley Park, who created reports based on this material by disguising its origin as signals traffic.

One item passed was "advance warning to develop tanks with stronger shells in the light of German armament reports.

Analysts deduced the northern and southern attack routes, and a report based on this transcript was passed through official channels to Stalin.

In addition to German Abwehr, Sicherheitsdienst (SD), Luftwaffe, naval, railway, Army group and OKW messages, GC&CS intercepted and decrypted Yugoslav partisan communications with Comintern and with the Soviet Union.

[17][21] Yuri Modin, the Russian MGB (later KGB) Controller in London, claims that Cairncross gave him details of nuclear arms to be stationed with NATO in West Germany.

[17] For security, the residence (or rezidentura, the local Soviet intelligence station) temporarily stopped contact with him, allowed him to continue to report monthly his situation with appropriate signals, and planned a follow-up meeting on 23 January 1952.

[17] The Soviets developed an exfiltration plan for Cairncross including funds, documents, and communication methods while living in other countries.

[24] While this did not include technical data, it provided the Soviet Union evidence that the British were considering the production of atomic weapons.

The identity of the infamous 'fifth man' in the Cambridge Five remained a mystery outside intelligence circles until 1990, when KGB defector Oleg Gordievsky confirmed Cairncross publicly.

Although he knew Anthony Blunt at Cambridge, Guy Burgess socially (and had a dislike of both of them), Donald Maclean from the Foreign Office and Kim Philby from MI6, he claimed not to have been aware that they were also passing secrets to the Russians.

The news was widely publicized leading many to surmise that he was in fact the "fifth man", a designation which would be confirmed in 1989 by Gordievsky, who had defected to Britain.

After the book was published, former KGB controller Yuri Modin denied ever having named Rothschild as "any kind of Soviet agent".

Modin's own book's title clarifies the name of the fifth man: My Five Cambridge Friends: Burgess, Maclean, Philby, Blunt, and Cairncross.

A review of the 2019 book by Chris Smith, The Last Cambridge Spy: John Cairncross, Bletchley Park Codebreaker and Soviet Double Agent, proposes this view.

The review adds that unlike the other four, described as "privileged" and as "haute bourgeois" by another book (A Spy Named Orphan: The Enigma of Donald Maclean), he was "lower middle class" with a heavy Scottish accent.

In 1981, Prime Minister Margaret Thatcher informed parliament that Cairncross was a Soviet agent and was living with his wife in the west of England while he wrote his memoirs.

In 2001, writer Rupert Allason lost a court case in which he claimed to have ghostwritten The Enigma Spy in return for copyright and 50% of the book proceeds.

[40] Cairncross is depicted in part three of the 2003 BBC TV series Cambridge Spies, where he appears reluctant to continue passing Bletchley Park data to the Russians for fear that the Red Army was heavily penetrated by Abwehr (German intelligence) and by Eastern Front military intelligence under General Gehlen.