KGB Double Agent Oleg Gordievsky died at the age of 86.
Gordievsky would be the most valuable spy of Great Britain in living memory in the Russian intelligence services.
Terrorism fighting police helps the coroner, but his death is not treated as suspicious.
He died peacefully in his house in Surrey, reports the BBC.
Gordievsky, a colonel in the Russian KGB, worked as a double agent for many years and passed on vital intelligence to both Britain Mi6 and MI5.
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