defector

A person of intelligence value who goes over to a foreign service, usually in exchange for asylum.

The CIA glossary defines a defector as a person of intelligence value who voluntarily goes over to a foreign service in exchange for asylum. The International Spy Museum distinguishes from this the walk-in and the agent in place, who stays in post instead of fleeing.^1,2

An example: Vasiliy Mitrokhin, a senior archivist in the KGB’s foreign intelligence service from 1972 to 1984, secretly copied files and smuggled notes out of the building. In 1992 the British SIS brought him, his family and his archive to the United Kingdom. In 2014 the Churchill Archives Centre in Cambridge opened his papers to the public – for now only the redacted Russian-language versions.³

Another example: the KGB officer Oleg Gordievsky, an agent of the British SIS since 1973, was spirited out to the West after being summoned to Moscow in 1985 and falling under suspicion – on 21 July 1985 he crossed the Finnish border hidden in the boot of a car (see exfiltration).⁴