The Tommy Sheridan perjury trial has been shown a 40-minute video in which he allegedly admits to twice visiting a swingers' club.
In the tape, a voice identified by ex-colleague Carolyn Leckie as Mr Sheridan - is heard saying: "Done it once before in '96 and went back in 2003".
Mr Sheridan and his wife Gail, both 46, are on trial at the High Court in Glasgow accused of perjury.
They deny lying during his defamation case against the News of the World.
Mr Sheridan won £200,000 in damages after the newspaper printed allegations about his private life, claiming that he was an adulterer who had visited a swingers' club.
After a police investigation, Mr and Mrs Sheridan were charged with perjury.
The video tape shown in court is said to have emerged after Mr Sheridan's court victory.
When it was played for the jury, a man's voice could be heard saying: "Done it once before in '96 and went back in 2003... at the time it was a great idea".
Former Scottish Socialist Party (SSP) MSP and colleague, Carolyn Leckie, said the voice on the tape making the admission was that of Mr Sheridan.
She identified another man on the tape as George McNeilage, the best man at Mr Sheridan's wedding.
The trial has already heard from a number of witnesses who said Mr Sheridan told an SSP meeting in 2004 that he had visited a swingers' club.
On the tape, the man identified as Mr Sheridan is heard discussing a woman who had taken an overdose and was in hospital because she had apparently been harassed by journalists, saying she should admit to an affair with him.
The man says: "Duncan thinks he is doing the honourable thing to save this lassie's life. Goes to the News of the World and fires in...Katrine Trolle's name."
The man alleged to be Mr Sheridan then continued: "That's the lassie from Dundee, right?"
The man said to be Mr McNeilage said: "Her that was in Cupid's with...you and...Anvar Khan."
'Look like liars'
The man said to be Mr Sheridan replied: "Absolutely, absolutely he goes and fires it."
He went on to describe Ms Trolle as "solid", saying "she will never admit it."
During earlier evidence, Ms Leckie told how she was alerted to an article in the News of the World in November 2004 about an MSP visiting a swingers club with journalist Anvar Khan.
A number of party members met at her partner Alan McCombes flat in Glasgow amid suspicion the politician was Mr Sheridan.
Miss Leckie said: "We were trying to avoid a formal meeting. We thought that we could persuade him (Mr Sheridan) to deal with it in a way that was honest and avoid what happened subsequently."
She added: "I was concerned about the impact on the SSP. We had been successful (at the elections).
"It was good times and it was like we had been hit by a rocket."
The former MSP said there was some discussion about whether to lie in court.
She told the jury: "There were some people arguing that we should lie at the defamation action. There was a vote and there were 13 for telling the truth and two people for not telling the truth and one person abstained."
She also criticised the way Mr Sheridan's wife had become involved in the affair.
She told the court: "We thought resigning as convener was enough, that it was not our business for us to give that information to anyone else.
"I probably have regrets about that now - particularly I think it's absolutely disgusting that Tommy has dragged Gail through this."
Miss Leckie was also shown a newspaper article in which Mr Sheridan branded her a "scab" after he won the defamation case.
She told the court: "I was absolutely gutted, angry. All my life I have organised strikes, supported them, been a socialist and never crossed a picket line.
"It really was a dreadful slur. It was the worst insult you could throw. It was calculated to be."
The ex-MSP also denied being part of a "cabal" to oust Mr Sheridan.
She said: "It is a conspiracy theory. It is nonsense. We were devastated this all happened, absolutely devastated."
Miss Leckie was later asked by Mr Sheridan's defence counsel Margaret Scott if she was angry that Mr Sheridan had won his libel trial against the News of the World.
She had been one of a number of SSP member who gave evidence in the case.
Miss Leckie said: "I think it was unjust because it made us look like liars. I was angry with Tommy Sheridan having set us up and accused us of perjury.
"It cast us as liars and plunged the SSP into crisis."
It is alleged that Mr Sheridan made false statements as a witness in his defamation action against the News of the World on 21 July 2006.
He also denies another charge of attempting to persuade a witness to commit perjury shortly before the 23-day legal action got under way.
Mrs Sheridan denies making false statements on 31 July 2006, after being sworn in as a witness in the civil jury trial at the Court of Session in Edinburgh.
The trial, before judge Lord Bracadale, is due to last between two and three months and is expected to become the longest perjury case in Scottish legal history.