According to "Spycatcher" by Peter Wright a soviet mole was a part of MI6 from the postwar period through the 1970's. While I do not beleive that this account has ever been officialy verified, Wright's thesis fits well with known facts and explains numerous intelligence failures throughout this period.

Due to placement in counterintelligence this alleged spy was in a position to provide the USSR with critical intelligence. The Philby defection in 1951 had begun a strain on US - UK intel relations, and it is likely that the presence of a mole in MI6 was what resulted in Philby escaping arrest. In the latter 1960's CIA began to collect evidence that operations were being compromised. This eventually resulted in the US cutting off most sharing of intelligence data with the UK.

Wright believed that he had ultimately identified the agent. This was an inherently difficult task as it is not easy to investigate someone placed near the top of counterintelligence without warning them that they are under suspicion. And indeed the individual (who is not named in Wright's book) is reported to have escaped.

Spycatcher Peter Wright