Members of the notorious Cambridge spy ring were described by their Russian handlers as "hopeless drunks" who were incapable of keeping secrets, newly-released files suggest.
Documents from the Mitrokhin Archive - described by the FBI as the most complete intelligence ever received from any source - have been opened to the public for the first time after being kept at a secret location for more than 20 years.
Major Vasili Mitrokhin smuggled the information out of Soviet archives during 12 years working for the KGB before defecting to Britain in 1992.
ITV News Diplomatic Correspondent John Ray reports:
Among the thousands of pages of documents are profiles outlining the characteristics of Britons who spied for the Soviet Union.
More than 200 names of British people who contributed to Soviet intelligence in some way are listed in the document's appendix.
They include references to Donald Duart Maclean and Guy Burgess, two of the five men recruited while studying at the University of Cambridge during the 1930s.
A short passage buried among more than 100 pages of intelligence, describes Burgess as a man "constantly under the influence of alcohol".
Moving on to Maclean, the note describes him as a man who - worryingly for a spy - was "not very good at keeping secrets", adding that he was "constantly drunk".