MI5 'struggled' with Soviet spies
British intelligence could barely cope with the number of Soviet spies in the UK during the Cold War, according to an official history of MI5, BBC reported.
The book found that MI5 did not get to grips with Soviet espionage in Britain until the early 1970s.
The history was written by Professor Christopher Andrew, who insisted he was given complete access to MI5's files.
The Defence Of The Realm also reveals numerous attempts by politicians to use MI5 for their own ends.
Professor Andrew said it was not until 1971, when 100 Soviet diplomats were expelled from Britain, that MI5 contained KGB and Soviet bloc intelligence activities in the UK.
He was given access to all 400,000 files created by MI5 since it was founded in 1909. But the agency did limit what he could publish.
Professor Andrew said documents also showed that MI5 was under pressure from Margaret Thatcher to help her in industrial disputes, such as the miner's strike.
According to the book, the prime minister wanted agents to identify all union "wreckers" who were stirring up industrial action.
Professor Andrew said Mrs Thatcher's demand was resisted because they were not genuine subversives.
The authorised history reveals that the Ministry of Defence pressed for information it could use against peace groups like CND, which were being monitored in the 1980s.
Thousands of files were opened on left wing activists during the Cold War.
The book also details MI5's failure over many years to break the Cambridge spy ring that worked for the Soviet Union.
MI5 did not realise until 1982 that John Cairncross was the fifth member of the ring, even though he had confessed to being a spy in 1964.
The book has been published to mark MI5's 100th anniversary.
Professor Andrew said he was grateful to MI5 for allowing him access to the files.
He said: "This is the first, thanks to MI5. The first ever history of any of the world's major intelligence agencies in which an outside historian has been given access to just about all of the files and been as able to come up to the present day."