Oleg Gordievsky, the long-standing KGB double agent who defected to Britain, has died aged 86.
Gordievsky was said to be Britain’s most valuable spy in living memory inside Russia’s intelligence agencies.
Counter-terrorism police are assisting the coroner, but his death is not being treated as suspicious.
He died peacefully at his home in Surrey, the BBC understands.
Gordievsky, a colonel in Russia’s KGB, spent many years as a double agent, passing vital intelligence to both Britain’s MI6 and MI5.
He has lived in Surrey under police protection since Moscow became suspicious of him in 1985 and he narrowly escaped arrest, trial and a firing squad by getting smuggled across the border into Finland in the boot of a car.
Two years earlier, as the KGB resident in London at the height of the Cold War, he warned his British handlers that Moscow had become so paranoid about an imaginary surprise attack by the West that the Soviet Union began making preparations to strike first.
As a result of his tip-off, NATO curtailed its military exercise codenamed Able Archer, and the crisis was averted.
