He was one spy who left many others out in the cold.

Declassified British government files reveal how the 1985 defection of a senior KGB agent set off a domino-chain of diplomatic retribution that officials feared could collapse relations with the Soviet Union just as the Cold War was beginning to thaw.

As the two countries sent each other's spies packing, Britain's ambassador in Moscow, Bryan Cartledge, warned graphically of the danger posed by a spiral of tit-for-tat expulsions. He cabled London: "Never engage in a pissing match with a skunk: He possesses important natural advantages."

The spat was triggered by the defection of KGB spy Oleg Gordievsky. For more than a decade, he leaked Kremlin secrets to London; when he came under suspicion, British agents smuggled him out of Russia in the trunk of a car.

Intelligence historians consider Gordievsky — code-named Hetman — one of the era's most important spies.

The papers, released by the National Archives under the "30-year rule" for declassifying secret documents, show Gordievsky was considered so valuable that Prime Minister Margaret Thatcher approved an attempt to cut a deal with Moscow: If Gordievsky's wife and daughters were allowed to join him in London, Britain would not expel all the KGB agents he had exposed.

Moscow rejected the offer, and Thatcher ordered the expulsion of 25 Russians, despite objections from Foreign Secretary Geoffrey Howe. He wanted the number kept to nine, fearing a mass expulsion could scuttle relations just as reforming Soviet leader Mikhail Gorbachev was easing the stalemate between Russia and the West.

In a memo, Howe said that "the Russians would be likely to freeze the Anglo/Soviet dialogue, probably for as much as two or three years."

Moscow responded by expelling 25 Britons, sparking a second round in which each side kicked out six more officials. But, despite Howe's fears, diplomatic relations were never severed.

The files reveal that Thatcher agreed to a Foreign Office recommendation to "draw a line under the Gordievsky episode" and not expel Czech, Bulgarian and East German agents the defector had unmasked.

Gorbachev and Thatcher went on to form a constructive relationship. The 1985 documents include a warm exchange of birthday greetings between the two leaders.

Gordievsky's family was kept under 24-hour KGB surveillance for six years before being allowed to join him in England in 1991.

In 2007, Queen Elizabeth II appointed Gordievsky a Companion of the Order of St. Michael and St. George.

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