Three political allies who gave evidence in support of Tommy Sheridan during his defamation action against a Sunday newspaper have now been charged by police, it emerged yesterday.

Jock Penman, 58, and 39-year-old Graham McIver were arrested by officers investigating allegations of perjury, police said. Yesterday's development follows the arrest and charging of Rosemary Byrne, 59, the former Scottish Socialist Party MSP, earlier this week.

Mr Sheridan, 43, was charged with perjury by Lothian and Borders Police shortly before Christmas last year. His wife, Gail, and father-in-law Gus Healy are due to be interviewed at an Edinburgh police station next week.

Last night, a spokesman for his Solidarity party repeated the claim that Mr Sheridan and his supporters were the victim of a "political witch-hunt". One source close to the party said the police tactics were like "toothpaste-squeezing" to put further pressure on Mr Sheridan.

Mr Sheridan, a former Glasgow MSP, won his defamation case against the Rupert Murdoch-owned News of the World in August, 2006, and was awarded £200,000 in damages.

The tabloid had branded him an adulterer and claimed he had visited Cupids swingers' club in Manchester and had taken part in group sex sessions. Mr Sheridan has always denied the claims.

The trial judge, Lord Turnbull, voiced his concerns about the contradictory nature of the evidence given during the trial and a month later, the Crown Office ordered Lothian and Borders Police to launch a perjury investigation.

A spokeswoman for the force said yesterday: "In addition to the 43-year-old man who was charged with perjury in December, a further two men, who are 58 years old and 39 years old, and a woman who is 59 years old, have been charged and released. A report will be sent to the procurator-fiscal and inquiries are continuing with regards to others."

Mr Sheridan was first elected to Holyrood in 1999, when he was the sole MSP for the Scottish Socialist Party (SSP). He was joined by five others in 2003, including Rosemary Byrne.

During the trial, Mr Sheridan called Ms Byrne as a witness and she denied claims that he had admitted visiting the swingers' club during an SSP meeting on November 9, 2004, to discuss the newspaper's allegations.

Mr Penman, the former SSP organiser for Mid-Scotland and Fife, told the Court of Session that Mr Sheridan had denied visiting Cupids and instead put the allegations down to "workplace gossip".

Mr McIver, another SSP member, told the court that other people at the meeting - including three SSP MSPs - who claimed Mr Sheridan had confessed, were "mistaken".

Shortly after his court victory, Mr Sheridan and Ms Byrne left the SSP and set up a new party, Solidarity. They both lost their Holyrood seats at last year's election, as did their former SSP colleagues.

A spokesman for Solidarity, Jim Monaghan, insisted last night that the party was relaxed about the latest turn of events.

He said: "We consider this to be a political witch-hunt. I don't think we have anything to worry about.

"We believe it's just police intimidation and it's to put pressure on members of Solidarity and to put pressure on Tommy Sheridan.

"We expected this as part of an ongoing pressure campaign. Our members have done nothing wrong and have nothing to fear in a court at all."

Mr Sheridan was arrested by police on December 16, just minutes after finishing his Talk 107 radio phone-in show.

After being charged at Edinburgh's Gayfield police station, he said: "I believe this whole farcical inquiry has usurped an incredible amount of public resources.

"It has been orchestrated and influenced by the powerful reach of the Murdoch empire and I believe that I am the victim of a witch-hunt from the Murdoch empire. I will prove my innocence in the fullness of time."

Aamer Anwar, Mr Sheridan's lawyer, said: "As inquiries are ongoing, it would be inappropriate to comment, other than to say Mr Sheridan maintains his innocence."