Tommy Sheridan came out fighting today as he attempted to shrug off claims that he had been caught on a secret video recording "confessing" to cheating on his wife and visiting a sex club.

Just a day after prosecutors in Scotland announced a police investigation into allegations of perjury by witnesses during Mr Sheridan's five-week libel trial against The News of the World, the socialist politician launched a bitter attack on the newspaper and said that he was ready for a new battle.

In typically belligerent style, Mr Sheridan, whose £200,000 victory over the tabloid this summer turned him into Scotland's most famous politician, said that the latest allegations against him were "garbage and lies, cococted to try and attack socialists".

Speaking at a press conference in Dundee to promote his new political party, Solidarity, the former leader of the Scottish Socialist Party (SSP) insisted that he had never been inside the house where the video is alleged to have been made and said that the recording had been faked, apparently by splicing segments of his voice with at least one other person's.

He said that he had listened to the recording, the full transcripts of which were made available by The News of the World this weekend on a telephone hotline and also via the newspaper's website, but dismissed claims by alleged experts, including, reportedly, the man who has verified the voice of Osama bin Laden for the White House, that it was authentic.

The 40-minute conversation, allegedly taped in mid-November 2004 when the first allegations about Mr Sheridan's private life were being made by The News of the World, can be heard over pictures filmed on a camcorder hidden beneath a pile of tiles by George McNeilage, best man at the politician's wedding in 2000.

Mr Sheridan claimed yesterday that his former friend and classmate had been paid £20,000 by the newspaper.

He said: "I know that I wasn't in the house in question, I've never been in the house in question so I know it [the tape] was manufactured, I know it was concocted. The fact of the matter is that with computer graphics as they are, anyone's voice can be spliced into a set of sentences and even made to sound like someone... Not only is there more than once voice on that tape but it has clearly been spliced on several occasions. That is a concoction, that is what that tape is."

He added: "When I've listened to it I've got to say maybe that sounds like me — and then you can hear what sounds like running water, you can here what is called the 'white noise' in the background... segments of my voice have been used but certainly someone else's voice has been used as well."

The Crown Office in Edinburgh announced on Monday that it had instructed police to begin a formal perjury investigation into evidence given by all witnesses during the libel trial, which ended with a spectacular victory for Mr Sheridan.

At the time The News of the World described the jury's verdict as "perverse". It had claimed in a series of articles that Mr Sheridan was a serial sex cheat who participated in orgies and visited a swingers' club.

Mr Sheridan, who has since left the SSP to form a new socialist party, Solidarity, said today that those who cooperated with "the Murdoch empire" should "hang their heads in shame...they are now clearly in bed with the enemy of socialism and the enemy of trade union values and trade union principles".

He called for clemency to be shown on anyone found to have lied in court. Referring to the editor of The News of the World in Scotland, he said: "I don't like Bob Bird but I don't want him to go to jail... Cleaning the public toilets for a couple of years would be of more service than sending him to one of our overcrowded jails."

Those who gave evidence against him included three MSPs, and it is believed that their evidence will also be scrutinised during the investigation, which is expected to take months.

The News of the World have said that they are "100 per cent" confident that the recording is genuine. Insisting that he had nothing to fear from the investigation, Mr Sheridan said: "If you [the News of the World] want a fight, come ahead, we'll take you on."

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