Labour leader Jeremy Corbyn, as well as key party hardliners John McDonnell and Ken Livingstone were part of a group of at least 15 senior Labour figures who regularly met with Eastern bloc agents, it is claimed.
Jan Sarkocy, a former Czechoslovak spy, described the MPs as "great sources" for the Czech spy service, the Statni Bezpecnost, and Russia's KGB, reported The Telegraph.
A spokesman for the Labour Party said: "The former Cold War agent Jan Sarkocy is a fantasist, whose claims are entirely false and becoming more absurd by the day. These ridiculous smears should be given no credence whatsoever."
The latest claims come after files were found in Statni Bezpecnost archives in Prague this week which revealed the Islington North MP met with a Czech agent three times in the 1980s and was given the codename 'COB'.
Labour described the fresh claims as "absurd", while former Mayor of London Ken Livingstone said they were a "tissue of lies". Shadow chancellor John McDonnell dismissed the claims as "ridiculous".
Corbyn admits meeting Czech agent Sarkocy on a small number of occasions, but denies supplying information to a foreign power.
Sarkocy worked for the Statni Bezpecnost at Czechoslovakia's London embassy from 1986 to 1989, when he was expelled.
He said there was "no question" that Corbyn and the other Labour sources knew that he was a spy.
He described Livingstone as a "good boy" who liked to drink whisky.
He labelled Corbyn as a "smart and polite guy" who was a communist "from the inside", which went "right back to his parents who held similar beliefs".
The former Czech spy alleges that at one meeting Corbyn warned him about British operations by handing him a newspaper cutting showing MI5 was investigating suspected spies.
Sarkocy said: "From Mr Corbyn we obtained good information that we could use. It is like a crossword."
The former agent added McDonnell, who was deputy leader of the London's local government, the GLC, under Livingstone until becoming an MP in 1997, regularly met a Soviet agent named P. Tibor.
A spokesman for McDonnell said: "These are ridiculous and false allegations. John never met any Czechoslovak or Soviet agent, nor visited the Soviet or Russian embassy and has only visited Guildford once in his life, which was last year for a Labour Party public meeting."
Ken Livingstone said: "Information? What information would I have? I was a backbench MP and didn't have access to anything. It's just a tissue of lies."
He added: "Whisky is not my drink of choice, it's brandy. I have no recollection of meeting anyone from the Czech embassy at all. I do remember meeting people from the Russian Embassy."