Members of the Cambridge Five spy ring were regarded by their Soviet 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 today 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.
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".
Written in Russian, 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.
It was believed that he had told one of his lovers and his brother about his work as a Soviet agent while he was the worse for wear, the file adds.
However, the notes also provide an insight into the extent of the group's activity as they helped the KGB penetrate the UK's intelligence network at the highest level.
They describe how Burgess alone handed over 389 top secret documents to the KGB in the first six months of 1945 along with a further 168 in December 1949.
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.
After being recruited during their studies, the group went on to occupy positions within the Foreign Office, MI5 and the Secret Intelligence Service (SIS).
Shortly before the end of the war, Philby was promoted to head of the SIS's anti-Soviet section - meaning he was in charge of running operations against the Soviets while operating as a KGB agent.
Mitrokhin was a senior archivist in the KGB's foreign intelligence HQ and had unlimited access to thousands of files from a global network of spies and intelligence gathering operations.
He became disillusioned with the Communist Party of the Soviet Union and began handwriting notes from the files which he believed would be of use to foreign intelligence.
His defection was regarded as a major coup and provided an insight into the extent of Soviet intelligence operations throughout the cold war.
Throughout his life he made it clear he wanted his files opened to the public and following his death in 2004 his family worked with the Churchill Archive Centre in Cambridge to realise this wish.
His handwritten notes made in school notebooks remain classified and some information has been redacted.
But 19 out of 33 box files containing typewritten versions of his notes, all in Russian, can be viewed by visitors to the archive centre.