Documents from the Mitrokhin Archive -described by the FBI as the most complete intelligence ever received from any source - have this week been opened to the public for the first time after being kept at a secret location for more than 20 years.
Soviet official Major Vasili Mitrokhin smuggled the information out of state archives during 12 years working for the KGB before defecting to Britain in 1992.
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".
It goes on to recount one occasion when Burgess drunkenly risked exposing his double identity.
"Once on his way out of a pub, he managed to drop one of the files of documents he had taken from the Foreign Office on the pavement," translator Svetlana Lokhova explained.
Moving on to Maclean, the note describes him as a man who - worryingly for a spy -was "not very good at keeping secrets". It adds that he was "constantly drunk" and binged on alcohol.
However, the notes also provide an insight into the extent of the group's activity as they helped the KGB penetrate UK intelligence.
Along with Kim Philby, Anthony Blunt and a fifth man, widely believed to be John Cairncross, the Cambridge Five passed information about the UK to the Soviet Union throughout the Second World War and into at least the 1950s.