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."