In the course of nine years, Blake is said to have betrayed details of some forty MI6 agents to the KGB, destroying most of MI6's operations in Eastern Europe, although this remains unsubstantiated. Blake later said of this: "I don't know what I handed over because it was so much." In the same TV interview, Blake claimed to have betrayed 500 British agents. In 1959 Blake became aware of a Central Intelligence Agency mole inside GRU, and was possibly instrumental in exposing P. S. Popov, who was executed in 1960.