Labour has denied allegations Jeremy Corbyn had been either a collaborator or agent of the Communist regime in Czechoslovakia in the 1980s.
The Labour leader’s spokesman said the claims had come from a single source and were “absurd and hallucinogenic”.
It comes as the German authorities said it had found no records of a Stasi file on Mr Corbyn in “recent searches,” following press speculation.
Ex-Czech agent Jan Sarkocy claims he met Mr Corbyn several times in London.
Asked to describe the then backbench MP’s relationship with the Czech secret police, the StB, Mr Sarkocy told BBC News: “It’s very hard to say, but he was open.”
Pressed on whether Mr Corbyn knew Mr Sarkocy was a member of the secret police, he said: “Yeah, absolutely, because at the time, you know, everyone knew who is who at the Embassy.”
Mr Sarkocy, who was interviewed by the BBC in Bratislava, Slovakia, described Mr Corbyn as a “very, very good source”.
Mr Corbyn’s spokesman said the Labour leader recalled one meeting with an unnamed Czech diplomat to discuss disarmament in the 1980s but disputed other reported encounters, accusing Mr Sarkocy of making a series of outlandish claims.
He said claims he was “sympathetic” to the Soviet-backed Czechoslovak regime were false.
Mr Corbyn has suggested a series of front page headlines over the past week about his contacts with Mr Sarkocy, who worked in the Czechoslovak embassy in London in the 1980s, were politically motivated.
In a Facebook message on Tuesday, he suggested that the coverage, based on a file which the Communist-era Czech intelligence agency compiled on Mr Corbyn, was driven by right-wing newspaper owners who were “worried” about the prospect of a Labour government.
The director of the Czech Security Service Archive has said the files suggest Mr Corbyn was seen as a possible “contact” for the authorities but there was no evidence to suggest he was an informant or collaborator.
The body which runs the archives of the former East Germany’s secret police, the Stasi, said it had not found “any records” on the Labour leader in “recent searches”.
The Federal Commissioner for the Stasi records has now scotched press speculation that the much-feared secret police compiled a dossier on Mr Corbyn after he toured the country with his then girlfriend – Labour MP and shadow home secretary Diane Abbott – in the 1970s.
In a statement, the Federal Commissioner said: “The most recent researches in the written records of the Ministry for State Security of East Germany have not produced any records or any other information on Jeremy Corbyn or Diane Abbott.”
It has been reported that East German agents targeted left-wing politicians and groups, infiltrating Labour Action for Peace, a group in which Mr Corbyn was active in during the 1980s.
Speaking after Prime Minister’s Questions, a Labour spokesman said there was nothing to indicate that Mr Corbyn had been aware of this.
More broadly, he said the press had had “creative fun” with the spy stories but the claims were based on a single source and had been denied by the Czech intelligence archives.
Mr Corbyn, who is said to have “good records” of his contacts from the period, remembered meeting one Czechoslovak official to discuss the prospect for peace and disarmament, a discussion that took place at a time when there were growing hopes of detente between the Soviet Union and the United States.
The Labour leader was “not embarrassed” to have met a wide range of people from both Warsaw Pact and Nato countries during the Cold War, his spokesman said, but some of the alleged meetings were “demonstrably false”.
The MP, he said, had been at a socialist conference in Chesterfield on 24 October 1987, one of the dates in which he was reported to have met Mr Sarkocy.
The Labour leader, meanwhile, has called on Tory MP Ben Bradley to apologise or face legal action for a post on Twitter in which he claimed Mr Corbyn had “sold British secrets to Communist spies”.
Mr Corbyn’s lawyers have sent a letter to the Mansfield MP urging him to confirm in writing that the “defamatory statement” – which they say is damaging and potentially harmful to the Labour leader’s reputation – will not be repeated in any form.
Mr Bradley, who had already deleted the tweet following criticism on social media, is also being urged to make a donation to a charity of Jeremy Corbyn’s choice “in lieu of damages”.