Upon his release, received a transfer to MI6 in order to work as a secret agent. Despite a standing rule that only citizens of British parentage were eligible for acceptance into MI6, Rotterdam born Blake was accepted anyway.

After being exposed in 1961 by a Polish defector, Blake was arrested by SIS agents. After thorough questioning, Blake provided a full confession and was subsequently charged with violating the Official Secrets Act. British agents were horrified to discover that Blake was one of the most damaging spies in Britain's history and he was quickly convicted and sentenced to 42 years in Wormwood Scrubs Prison.

Escaped from Wormwood in 1966, breaking out of his cell window and then climbing a makeshift ladder supplied by a former prison colleague, Sean Bourke, who then drove Blake to safety. Making his way through East Germany, he eventually would up in Moscow where he would meet with fellow spies Gordon Lonsdale, Kim Philby and Donald MacLean.