Image caption Sheridan's lawyer will ask police to consider investigating whether Mr Coulson committed perjury

A Labour MP told the Commons the News of the World scandal could make Tommy Sheridan's perjury conviction unsound.

During Sheridan's perjury trial last year, former News of the World editor Andy Coulson denied knowledge of his paper paying corrupt police officers.

But it has now been reported that newly found e-mails suggest Mr Coulson authorised payments to police.

Sheridan's lawyer is to ask Strathclyde Police to begin an inquiry into whether Mr Coulson committed perjury.

During an emergency Westminster debate on the News of the World phone hacking scandal, Tom Watson, former Parliamentary Under-Secretary of State at the Ministry of Defence, said he thought there was now a question over the reliability of the former MSP's conviction.

"I think the Sheridan trial was unsound and may need to be revisited," he said.

On Tuesday he told BBC Scotland that News of the World e-mails relating to Sheridan had been found in London, despite claims during the perjury trial that they had been lost in India.

Sheridan was convicted of lying during his successful defamation case against the News of the World in 2006.

The newspaper had claimed that the socialist politician was an adulterer who had visited a swingers' club.

'Dark arts'

BBC Scotland understands that on Thursday Sheridan's lawyer Aamar Anwar will ask Strathclyde Police to consider a perjury investigation into the evidence given by the former News of the World editor.

Mr Anwar is expected to be joined by the Labour MP Tom Watson at a press conference in Glasgow.

I don't accept there was a culture of phone hacking at the News of the World Andy Coulson giving evidence at the trial of Tommy Sheridan

Mr Watson and Mr Anwar are also expected to make documents public which suggest senior executives of News International, the owners of the News of the World, were previously involved in commissioning private detectives.

During the perjury trial at the High Court in Glasgow, Mr Coulson, who was was director of communications at Downing Street at the time, denied presiding over "dark arts".

He left the newspaper when the tabloid's royal editor, Clive Goodman, was jailed for conspiracy to access phone messages. Private investigator Glenn Mulcaire was imprisoned for six months on the same charge.

During his evidence, Mr Coulson denied knowing of Mr Mulcaire until the Goodman court case.

He told the court: "I don't accept there was a culture of phone hacking at the News of the World."

When asked by Sheridan, who was defending himself, whether his newspaper paid corrupt police officers, Mr Coulson answered "not to my knowledge".

"There is nothing I am not prepared to discuss in relation to my time at the News of the World," he said during heated exchanges with Sheridan.

Mr Coulson also denied he been given a pay-off to keep quiet by the News of the World.

No evidence

Prime Minister David Cameron has promised to set up a public inquiry into phone hacking at the News of the World.

Mr Coulson resigned from his government post in January, saying claims over phone hacking during his time as News of the World editor were making his job impossible.

But a Press Complaints Commission investigation in May 2007 found no evidence that Mr Coulson or anyone else at the paper had been aware of Goodman's activities and, that same month, he became Mr Cameron's director of communications.

Revelations involving the News of the World continue to emerge following the claims that the voicemail of murdered schoolgirl Milly Dowler was hacked.

Families of victims of the 7 July bombings in 2005 have complained that they may have had their phones hacked.

Police investigating hacking claims against the paper have also contacted the parents of murdered schoolgirls Holly Wells and Jessica Chapman.