Who Were the Cambridge Five?
The Crown: These Were the Real-Life Spies Who Made Up the Cambridge Five
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The idea of a "fifth man" persisted after the original four spies were discovered. In Blunt's confession, he named John Cairncross, who worked with the decryption team on Operation Ultra. Cairncross apparently passed the decrypted transcripts of German plans directly to the Soviets during World War II. He was rumoured to have passed nuclear intelligence as well, but his access and knowledge base make this particular rumour less likely than some of the others. Cairncross worked more independently than the other four men in the Cambridge spy ring and reportedly disliked the ones he had met, such as Blunt and Philby. He confessed to spying in 1951 when a note from Cairncross was found in Burgess's abandoned apartment, but he was never prosecuted.
Few images publicly exist of Cairncross, and, indeed, his involvement in the ring was a closely guarded secret outside of the intelligence community until 1990, when a KGB defector named him publicly and reported that he was one of the most valuable spies the KGB had ever had. Cairncross spent his postwar life in several countries, working in the US, Italy, and France in a variety of positions. Although he isn't named during the Blunt story arc on The Crown, Cairncross's spy activities have been portrayed on screen before: his time working with Operation Ultra was depicted in The Imitation Game, where he was played by Allen Leech.