Tommy Sheridan has said he is ready to "take on" the News of the World after it claimed it had a tape which showed the MSP admit visiting a sex club.
The socialist politician said the tape was concocted and dodgy and he had been the victim of a stitch-up from day one.
The member of the Scottish Parliament said: "If you want a fight, come ahead, we'll take you on."
The newspaper said the tape had been verified by four voice analysts who agreed Mr Sheridan was featured.
But Mr Sheridan, a Glasgow list MSP, remained defiant as he held a press conference in Dundee, and insisted the tape was manufactured.
"I saw with my own eyes when we made party political broadcasts what we can do with computer graphics as far as voices are concerned," he said.
The tape is reportedly a recording by George McNeilage, who was one of Mr Sheridan's best men at his wedding.
Mr Sheridan said it was a "badge of honour" to be attacked by the News of the World and Sun papers, adding: "They epitomise everything we oppose as socialists.
"The fact they have chosen to attack me personally and Solidarity shows that they fear what we stand for."
Mr Sheridan also had a tough message for those whom he claimed had "collaborated, colluded and co-operated" to attack him, adding: "They are now clearly in bed with the enemy of socialism."
The MSP even said he would not be surprised if MI5 had been involved.
"The state has a fine history of trying to destabilise and undermine socialist movements," he said.
"When the history of this whole episode is written about, I think you'll find that MI5 certainly was involved."
Bob Bird, Scottish editor of the News of the World, said: "It's a straight forensic examination now. He says the tape is a fake, it is bits and pieces spliced together.
"That is an easy thing to prove or disprove and we are happy to give it to the police. I'm sure they'll be able to prove that within a matter of days or weeks at the most."
Colin Fox, SSP national convenor, said: "It is time for Tommy to face the facts, tell the truth and apologise to those he has branded liars, cheats and plotters.
"The great damage he is inflicting on the socialist movement and his friends can be halted if he has the courage to tell the truth but time is running out."
He added: "He needs to accept that his claims are just not credible and that it is time to face facts and deal with the disaster he alone has created."
Newspaper editor Bob Bird said welcomed the inquiry decision
Details of the alleged recording were published 24 hours before the Crown Office instructed police to establish if witnesses lied during Mr Sheridan's recent defamation case against the News of the World.
It is understood the two developments are not directly linked.
The MSP won £200,000 in August after successfully suing the News of the World over allegations about his private life.
Mr Sheridan said he had taken legal advice on whether he could sue the paper following its latest allegations, but he said may not be able to fight it on a "no win no fee" contract after he dismissed his legal team in the first court case.
The 23-day case heard a series of witnesses give conflicting statements about alleged events and meetings involving Mr Sheridan.
Among those who testified against him were 11 members of the Scottish Socialist Party (SSP), who Mr Sheridan accused of trying to oust him from the party.
The members told the defamation trial that Mr Sheridan had admitted visiting a swingers club in Manchester at a party meeting in November 2004.
Four of Mr Sheridan's party colleagues backed his account of that discussion, insisting he had made no such admission.
The trial and its outcome led to public acrimony within the SSP which culminated in Mr Sheridan and another MSP, Rosemary Byrne, leaving to set up the breakaway left-wing party Solidarity.