Jack Jones was former leader of Transport and General Workers' Union
Trade union leader Jack Jones was briefly considered by the Soviet KGB as one of their agents, according to an official history of MI5.
The book says he only passed on Labour party documents, not secrets, and was last paid by the Russians in 1984.
Jones, who died earlier this year, always denied working for the KGB.
The book - Defence Of The Realm - also found that British intelligence could barely cope with the number of Soviet spies in the UK during the Cold War.
Jack Jones was leader of the Transport and General Workers' Union from 1969 to 1978, and a veteran of the Spanish Civil War.
The book says the head of MI5 told the cabinet secretary in 1985 that he last received money from his case officer the year before.
That case officer was Oleg Gordievsky, who was working for British intelligence.
Gordievsky had previously claimed that Jones had accepted money.
The book was written by Professor Christopher Andrew, who insisted he was given complete access to MI5's files.
He alleges British intelligence could barely cope with the number of Soviet spies in the UK during the Cold War, and that MI5 did not get to grips with Soviet espionage in Britain until the early 1970s.
Although Professor Andrew was given access to all 400,000 files created by MI5 since it was founded in 1909, the agency did limit what he could publish.
Prof Andrew said documents also showed MI5 was under pressure from Margaret Thatcher to help her in industrial disputes, such as the miners' strike.
According to the book, the prime minister wanted agents to identify all union "wreckers" who were stirring up industrial action.
Mrs Thatcher's demand was resisted because they were not genuine subversives, Prof Andrew said.
The authorised history reveals 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 the agency's 100th anniversary.
Other revelations include:
• MI5 infiltrated the Communist Party of Great Britain in the 1950s, using hidden microphones and covert methods to gain membership records.
• A number of union leaders and MPs worked for Soviet bloc intelligence agencies; and
• Politicians urged MI5 to spy on industrial and political opponents on many occasions.
The book also claims Bruce Kent, of the Campaign for Nuclear Disarmament, was watched by MI5 after he was suspected of being an anarchist.
"This was 1977, I was a quiet little parish priest somewhere, I think. I wasn't doing anything naughty to anybody.
"I don't know why they thought I was an anarchist, I was quite well behaved."
Prof 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."
He told Radio 4's Today programme: "If you do not allow your history to be written, there's not just a blank where the history should be, there's disinformation, there's conspiracy theory."