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.
As NATO carried out the exercise, known as Able Archer 83, the Soviets and their Warsaw Pact allies moved onto a war footing. Historians consider this to have been the closest moment to world war since the Cuban Missile Crisis in 1962.
Mr. Gordievsky was in a unique position to work both sides. He was able to persuade Moscow that an attack was not in fact imminent while also communicating Soviet fears to the British and the Americans. As a result, Mrs. Thatcher and Mr. Reagan pared back their language, and future military exercises were more limited.
All of this remained secret for years afterward, and in the meantime Mr. Gordievsky had to watch his own back. In 1985 he was recalled to Moscow, given drugs and interrogated. Someone, it seemed, had tipped off the K.G.B. to the presence of a high-ranking mole in London.
Lacking solid evidence, the Soviets placed him on leave. A few days later he appeared at 7 p.m. on a Moscow street corner, holding a shopping bag. A man soon passed, eating a candy bar. They locked eyes.
That was the signal to activate Operation Pimlico, an emergency extraction. Mr. Gordievsky shook his K.G.B. tail and then hurried to the Finnish border. Two British agents, a man and a woman, along with their baby, awaited him there in their Ford Sierra.
They placed him in the trunk, wrapped in a foil sheet to confuse heat detectors. When dogs at the border grew suspicious, the agents began to change the child’s diaper, filling the car with odors that threw the canines off Mr. Gordievsky’s scent.
When they were finally across, they played Jean Sibelius’s “Finlandia” symphony on the car’s sound system, a signal to Mr. Gordievsky that he was safe.
Back in Moscow, he was sentenced to death in absentia. That sentence has never been rescinded.
Oleg Antonovich Gordievsky was born on Oct. 10, 1938, in Moscow. His father, Anton, was an agent with the N.K.V.D., the forerunner of the K.G.B., and his mother, Olga, was a statistician. His father was a committed Communist, but his mother quietly reviled the party, an attitude that greatly influenced her son.
Still, there was no question where his future lay. He graduated from the elite Moscow State Institute of International Relations in 1961, and he joined the K.G.B. two years later.
After an initial posting in East Berlin, he did two tours at the Soviet Embassy in Copenhagen, with time in between to improve his spycraft.
But as he rose in the K.G.B. ranks, he also grew disillusioned with Communism. In Germany he had seen the newly erected Berlin Wall split families, and from afar he had watched the Soviet Union crush the Prague Spring movement of 1968.
Working on a suggestion from a double agent who was a former colleague of Mr. Gordievsky’s, British intelligence agents began to feel him out in Copenhagen. Once he turned, he was considered among the West’s prize assets — so prized that the Americans and even Mrs. Thatcher did not know his identity.
While continuing to provide intelligence to Moscow — bits of low-value information fed to him by his MI6 handlers — he helped Western governments uncover spies within their ranks. Among them were Arne Treholt, a Norwegian diplomat, and Michael Bettaney, a British counterintelligence officer who in 1983 tried to pass classified documents to Arkady Guk, the rezident at the time.
The ensuing scandal led the Soviets to recall Mr. Guk, opening the door for Mr. Gordievsky to replace him.
Both sides in the Cold War used moles like Mr. Gordievsky to hunt for spies. It was later revealed that his cover was blown by Aldrich Ames, a C.I.A. officer who began working for the Soviets in 1985. Mr. Gordievsky was one of the first double agents Mr. Ames exposed and one of the few who escaped; nearly a dozen others were executed.
After defecting, Mr. Gordievsky lived under an assumed name in Godalming but continued to advise British intelligence. As the Cold War wound down, he began writing under his own name, including articles for The Daily Telegraph and the books “KGB: The Inside Story” (1990), a collaboration with the historian Christopher Andrew, and “Next Stop Execution: The Autobiography of Oleg Gordievsky” (1995).
His story was also the subject of “The Spy and the Traitor: The Greatest Espionage Story of the Cold War” (2018), by Ben Macintyre.
Mr. Gordievsky’s first marriage, to Yelena Akopian, a fellow K.G.B. agent, ended in divorce. He married Leila Aliyeva in 1979.
After defecting, Mr. Gordievsky spent years trying to get the Soviet Union to allow his wife and their two daughters, Mariya and Anna, to join him. They arrived in Britain in 1991, but the couple soon divorced.
Mr. Gordievsky’s survivors include his daughters.
Beginning in the mid-2000s, Mr. Gordievsky raised alarms about the increasing authoritarian rule of President Vladimir V. Putin of Russia, including his deployment of a robust network of spies and subversives.
When his friend and fellow defector Alexander Litvinenko was fatally poisoned by Russian agents in London in 2006, Mr. Gordievsky began to fear for his life. When he fell ill and went into a temporary coma in 2008, he maintained that he, too, had been poisoned by Russian agents.
Mr. Gordievsky continued to warn about renewed Russian espionage, saying that Britain had naïvely lowered its defenses.
“It is easier than ever to work and recruit here,” he wrote in The Daily Telegraph in 2010. “If anything, the overall Russian espionage presence in Britain is now bigger and more active than in my time.”
