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.
Reference:
Spycatcher
Peter Wright