Longest-undetected spy

The longest known career as an undetected spy was achieved by Melita Norwood (née Sirnis; UK), who was known to Soviet intelligence as "Agent Hola". She was a secretary at the British Non-Ferrous Metals Research Association – an innocuous-sounding organisation that oversaw research related to various defence projects, including the British nuclear weapons programme. Norwood began passing classified information to the NKVD (the predecessor of the Soviet KGB) in 1935, and became a full agent in 1937. She continued to routinely pass intelligence to her KGB handlers until her retirement in 1972, around 37 years after her initial contact.

The highlight of Norwood's spying career came in the 1940s, when the British Non-Ferrous Metals Research Association handled much of the research relating to "Tube Alloys" (the British nuclear weapons programme). Along with Manhattan Project spy Klaus Fuchs, Norwood's intelligence is sometimes credited with having advanced the Soviet nuclear weapons programme by as much as two years. Norwood's value as an intelligence asset declined after her top-level clearance was revoked in 1951 due to her left-wing political connections, but she continued to pass on information when she could. She was held in such high regard by the KGB that she was presented with the Order of the Red Banner during a 1970 visit to Moscow and provided with a pension after her retirement.

Although British security services had been tipped off that Norwood was a security risk as early as 1965, they opted to simply work to keep her away from restricted material. The British intelligence agencies had been embarrassed by a number of high-profile security breaches, and didn't want to further harm their cold-war credibility by revealing another. Her activities were revealed to the public in September 1999 with the publication of The Mitrokhin Archive, an edited summary of the KGB archives copied by defector Vasili Mitrokhin.

Given her advanced age, the government declined to press charges against the then 87-year-old retired spy. She died at the age of 93 in 2005. It is probable that there are other spies who have had longer undetected careers, but by the very nature of their achievement, we don't know anything about them.