The information emerged after he faced a background check in response to an employment offer in Washington DC.
As part of the process, he revealed what had happened to his friend and member of the presidential inner circle Arthur M Schlesinger, Jr.
He went into further detail about his time as a sleeper agent in his memoir After Long Silence, in which he claimed to have leaked no official information aside from a paper he had written.
There is some dispute around this as according to The Telegraph, the KGB released a file in the late 1990s which showed he had released more under the codename Nigel.
The Crown season 3 is now available to stream on Netflix.