George Blake, who died in Russia on Saturday at the age of 98, was the last in a line of British spies whose secret work for the Soviet Union humiliated the intelligence establishment when it was discovered at the height of the Cold War.
Britain says he exposed the identities of hundreds of Western agents across Eastern Europe in the 1950s, some of whom were executed as a result of his treason.
His case was among the most notorious of the Cold War, alongside those of a separate ring of British double agents known as the Cambridge Five.