Oleg Gordievsky, who was the top K.G.B. agent in London until he defected to the West in 1985 and revealed himself as a longtime double agent for British intelligence — making him one of the most highly placed Western spies during the Cold War — was found dead at his home in Godalming, southwest of London, on March 4. He was 86.

The local police, who discovered his body, said that they did not believe foul play was involved but that an investigation was ongoing.

The British foreign intelligence agency, MI6, first recruited Mr. Gordievsky in 1974, when he was based in Copenhagen. In 1982 he moved to London, where the K.G.B. tasked him with seeding disinformation about Prime Minister Margaret Thatcher before the next year’s general election.

In practice he helped the British root out secret operatives and informants working for the Soviet Union. He kept up enough of a front to please his K.G.B. bosses in Moscow, who soon promoted him to rezident, or head agent, in Britain.

He also played a crucial role in preventing what could have become World War III.

By the early 1980s, the Soviets were convinced that the United States was planning a first-strike nuclear attack under the guise of a major NATO exercise, a suspicion underlined by President Ronald Reagan’s bellicose rhetoric.