When Coulson, the man at the centre of the News International phone hacking scandal, gave evidence at the 2010 trial of Scottish Socialist Party (SSP) leader Tommy Sheridan, he was still Cameron’s director of communications.

This is the third criminal investigation he faces, placing yet more pressure on the government over Cameron’s relations with Murdoch’s media empire.

Sheridan was on trial for perjury in connection with a successful defamation case he brought in 2006 against News International’s now defunct News of the World. He was found guilty and jailed for three years earlier this year.

Now Coulson is one of three News International witnesses who may be investigated as to whether they knowingly made false statements during the trial. Coulson told Glasgow High Court under oath, “I don’t accept there was a culture of phone hacking at the News of the World.”

Asked by Sheridan, who was conducting his own defence, “Did the News of the World pay corrupt police officers?” Coulson said, “Not to my knowledge.”

At the trial, Sheridan’s legal team requested access to emails sent between the Scottish News of the World and private investigator Glenn Mulcaire. The latter was jailed in 2007 for accessing personal mobile phones and selling information he garnered to News of the World.

Bob Bird, then editor of Scottish News of the World, also told a pre-trial hearing, “Our archived emails have been shipped to Mumbai and it's difficult to get anything that is more than six months old.” No emails were made available.

Sheridan’s lawyer, Aamer Anwar, has said, “Bob Bird gave evidence that emails could not be disclosed as they were lost in an archive in Mumbai. We now know this to be totally untrue…. The police spent three years and £2 million on investigating Tommy Sheridan. The Crown Office claims the prosecution was in the public interest and that nobody was above the law. We now demand a similar robust investigation of the serious allegations facing those who gave evidence at the trial.”

Anwar submitted a dossier to Strathclyde Police including information relating to statements made by Coulson, Bird and former Scottish News of the World news editor Douglas Wight during the 2010 trial. It included a page from Mulcaire’s notebook containing Sheridan’s mobile phone number and PIN, evidence transcripts from News of the World employees given at the trial, and extracts from a “blue book” of instructions to private detectives from News of the World journalists, including former editor and News International chief executive Rebekah Brooks.

Anwar was supported by Labour MP Tom Watson, one of the key figures in bringing News International’s murky relations to light. Watson stated in Parliament, “I think the Sheridan trial was unsound and may need to be revisited.”

The MP wrote to the information commissioner and established that News International’s email archive was never transferred to India. He told the BBC, “The jury was not in full possession of the facts. I think Tommy Sheridan was wronged. Had they been in possession of the email it might have materially affected the outcome of the trial.”

At the recent Westminster select committee hearings, another Labour MP, Jim Sheridan, put questions to Rebekah Brooks and James Murdoch, the chief executive of Rupert Murdoch’s global company News Corporation. He said to the junior Murdoch: “Bob Bird, whether deliberately or inadvertently, misled the jury in Tommy Sheridan's perjury trial. Your company has not disclosed internal emails that may aid the appeal of Mr Sheridan. Why is that?”

Murdoch claimed to have “no direct knowledge.” Brooks denied the email had been found. She stated “it was actually a problem with our suppliers in India, and there was no such retrieval.”

Sheridan insists that his phone was hacked, his car was bugged and that a tape used in evidence against him was manufactured. Light could be shed on these and many other areas by access to the missing email archive.

The possibility of Sheridan’s case being revisited also represents a major political crisis for the Scottish Socialist Party, whose leading personnel colluded with News of the World and the police to secure a guilty verdict against their former leader. Their response has been to campaign for Sheridan to remain in prison by rejecting any possibility of wrongdoing by News International, the British parent company of News of the World.

The SSP’s only article to date on the News of the World scandal, apart from a blog by a member of their youth section, was published July 7 and written by former Stalinist Ken Ferguson. Its sole purpose is to accuse Sheridan of “piggy backing on the only too real grief of real victims” of News of the World.

“Set against the pain and anguish inflicted on war widows, terror victims and the parents of murdered kids by the tabloid, attempts to harness the issue in the service of convicted perjurer Tommy Sheridan are simply pathetic,” he declares.

The Scottish Socialist Party must defend Sheridan’s conviction because of its collusion with Murdoch’s media empire and the police.

Sheridan was imprisoned for perjury in January this year after a trial motivated by his earlier victory in a 2006 libel suit against News of the World over articles alleging that he attended sex clubs and took drugs. Sheridan won damages of £200,000.

The intense and prurient attack by News of the World on Sheridan became the occasion for an unprincipled factional struggle to remove him from his leadership of the Scottish Socialist Party. In large measure, this was animated by a concern that Sheridan was endangering the lucrative political careers the party’s top personnel had built for themselves. At the time of the original trial, Sheridan was one of six SSP members of the Scottish parliament. Their personal successes helped finance the party’s apparatus.

An October 5, 2007 article in the Scotsman newspaper noted, for example, that the Scottish Socialist Party’s Frances Curran, Carolyn Leckie and Rosie Kane were the three highest claimants of personal expenses in the Holyrood parliament, all varying between £60,000 and £70,000 on top of their £53,000 salary. All three gave evidence against Sheridan.

The dispute came to a head on November 9, 2004 at a meeting in the SSP’s Glasgow headquarters of the party’s executive, at which Sheridan said he wanted to sue News of the World against the advice of the majority present. It was officially agreed that Sheridan would step down from his position and pursue the case privately. The party immediately began to leak claims to the media that Sheridan had admitted the truth of the News of the World’s allegations.

This initiative was led by the party’s policy coordinator, Alan McCombes, who lied to his party and covered his tracks by publicly refusing to release the alleged minutes of the meeting. He spent a brief stint in prison for his supposed refusal, but had in fact secretly issued a sworn affidavit to the Sunday Herald presenting his version of the meeting’s events.

In his recently published article, “Downfall: The Tommy Sheridan Story,” McCombes notes that he did not even inform Colin Fox, Sheridan’s replacement as Scottish Socialist Party convenor, of the affidavit, stating, “This wasn’t the time to burden Colin with that knowledge.”

Shortly after the November 9 meeting, the party’s North East regional organiser, Duncan Rowan, contacted News of the World and gave a taped interview at its offices. He gave the newspaper more information against Sheridan, including the name of Katrine Trolle, who gave evidence in both trials. During the 2010 trial it became clear that Trolle was on very friendly terms with a police officer investigating Sheridan.

Fiona Maguire, Rowan’s girlfriend, was at the time reported to have had an affair with Sheridan. She was paid £20,000 and given flying lessons by News of the World, having been cultivated by Douglas Wight for use against Sheridan.

Following Sheridan’s victory in the 2006 libel case, News of the World began preparing a counterattack for which it was above all reliant upon the SSP.

The party’s minutes secretary, Barbara Scott, accompanied by Kane and Leckie, submitted what they claimed to be the minutes from the November 9 meeting to the Lothian and Borders police.

Scottish Socialist Party member George McNeilage sold News of the World a video recording alleged, but never authenticated in court, to have been made of Sheridan in 2004. McNeilage pocketed at least £200,000 from News International for his services, along with various smaller payments authorised by Bird, as well as a holiday.

Largely based upon this information, Sheridan and his wife Gail were charged with perjury in 2007.

There is little wonder that the SSP does not want the Sheridan case to be revisited. There are a massive number of concealed documents that may now come to light. During Sheridan’s trial, for example, a News of the World document entitled “Sheridan expenses” was presented listing payments to witnesses. Most of it had been redacted.

The Scottish Socialist Party is riddled with police informers, while paid agents of Murdoch’s media empire enter and leave the organisation at will. This makes it a vehicle for all manner of provocations against the working class and an ideal conduit for the media and state intelligence services.