Jeremy Corbyn was at a socialist conference in Derbyshire at the exact time he was alleged to have been meeting a Czech spy in the House of Commons, his office has revealed.

The Labour leader’s spokesman said his records proved that he was 150 miles away in Chesterfield at 4pm on October 24, 1987 – the date when a former Communist agent alleged he’d met him in London.

That day was also a Saturday, when the Commons wasn’t open. And, in a further move to undermine the claim, Corbyn’s office said that his mother had died the day before and he was unlikely to have arranged a meeting with a diplomat.

The revelation proved how “more absurd and hallucinogenic” the allegations made against him had become, it added.

The fresh media fightback came as the archive for East Germany’s Stasi secret police revealed it had no files at all on Corbyn, contradicting newspaper suggestions that he ought to request their release.

Speaking in detail for the first time about the allegations, the Labour leader’s spokesman revealed that the one meeting with ex-spy Jan Sarkocy in 1986 had featured nothing more than a discussion about international relations and hopes of reductions in nuclear arsenals.

“He remembers that the discussion was about peace and disarmament and issues current at that time,” the spokesman said.

Corbyn’s office released details of his diary as he became more combative in refuting the unsubstantiated claims made against him in recent days by a string of newspapers.

On Wednesday morning, he demanded an apology and damages from Tory MP Ben Bradley, who had tweeted that the Labour leader had “sold British secrets to Communist spies”.

During Prime Minister’s Questions, Theresa May tried to highlight the spying allegations, making a joke about Corbyn and “blank cheques” on public spending, adding “I know he likes Czechs…”

Cabinet minister Liz Truss tweeted the joke as it happened, though No.10 refused to elaborate on it other than saying it was “a pun”.

But afterwards, Corbyn’s spokesman addressed the allegations in a lengthy briefing for the media.

“Jeremy recalls having one meeting with a Czechoslovak diplomat, in 1986 and that meeting was in the House of Commons,” he said.

“In the original documents, which remember were secret documents at the time, the agent who claims to have been at the meeting also claimed to have been at other meetings, and reported as fact, including having a meeting with Jeremy on the 24th October 1987 in London, in the House of Commons.

“As it happens we can confirm on that day Jeremy was in fact in Derbyshire, at the Chesterfield Socialist Conference. It was the day after his mother died.”

The socialist conference also featured local MP Tony Benn as well as Ken Livingstone.

“There is no possibility that he was at a meeting with a Czech diplomat, or someone posing as a Czech diplomat, in the House of Commons, on a Saturday, in London, at the same time.”

“So, it’s just another example of how what is reported as fact, and claimed even in the original documents, let alone in the completely surreal set of allegations made ever since, are demonstrably false as well as being utterly ridiculous. That’s another reason to doubt what was recorded in the files.”

However, the Sun’s political editor responded by revealing a fresh document from the archive with a different date for the meeting, listing a contact with ‘COB’ [Corbyn’s code name] two days earlier on October 22.

Referring to the one meeting that Corbyn has a record for, in 1986 in the House of Commons, his spokesman denied that he had in any way acted as an informant or collaborator.

“There is a record of the meeting…in 1986.  It took place at 4 o’clock in the afternoon and there was another meeting at 5 o’clock, just enough time to have a cup of tea.

“The meeting was 32 years ago. He remembers that the discussion was about peace and disarmament and issues current at that time – which in 1987 was about reduction in nuclear capacity on both sides.

“Two years earlier Margaret Thatcher was saying Gorbachev was a man should could do business with. The issue of détente, glasnost, perestroika and better relations between East and West was centre-stage.”

Meanwhile, the Stasi archive in Germany revealed that it had no files or ‘any other information’ on Corbyn – or on Diane Abbott (with whom he travelled through the country in the 1980s) at all.

Asked if Corbyn could list other meetings with other Eastern Block diplomats, the spokesman replied: “Jeremy has good records but he doesn’t have comprehensive records of every meeting he had in the last 35 years.

“But there’s no issue, he has met diplomats, political representatives, politicians, activists from all sides in all conflicts.

“We don’t regard that as anything to be embarrassed about, rather the opposite, it shows the extent of Jeremy’s commitment and involvement in international affairs and the attempt to resolve conflict.”

He added that Corbyn had met diplomats and activists of all kinds from many countries.

“He has met many ambassadors and during the Cold War period, as with any other time, and he met diplomats from East Bloc countries.

“The details of all those meetings are not recorded. Some of them are depending on his records.”

“Jeremy has always believed in international dialogue and negotiation that’s very well known, including with hostile states and friendly states and all manner of other states. He has always had a strong commitment to peace and disarmament.”

The spokesman added that the allegations of Corbyn being a paid informant were “transparently ridiculous and contradictory”.

“The head of the Czech security archive has dismissed the claim that in any way Jeremy Corbyn was a contact, informant, let alone collaborator or agent for the Czech security service. However those claims have continued to be reported in ever more absurd and hallucinogenic forms.”