The police investigation into possible perjury during the Tommy Sheridan libel trial is to be dramatically stepped up, The Herald has learned.

The Crown Office is understood to have given Lothian and Borders Police the go-ahead to prepare a full and detailed inquiry after receiving a preliminary report.

Sources close to the force said officers were now expected to interview a fresh round of witnesses and visit the Cupids swingers club in Manchester which was key to the trial. Some witnesses were informed of the step-change on Monday.

Mr Sheridan, the Solidarity MSP for Glasgow, won £200,000 damages from the News of the World last August after a jury decided he had been defamed.

The tabloid claimed the MSP was an adulterer and had visited the Cupids club after he was married.

Mr Sheridan always denied the claims, and conducted his own case at the Court of Session after sacking his legal team during the five-week trial.

The judge, Lord Turnbull, suggested the evidence heard in court was so contradictory that someone must have lied.

More than a dozen members of the Scottish Socialist Party which Mr Sheridan founded, including three MSPs, told the court he had confessed to them in 2004 that he was an adulterer.

However, Mr Sheridan, his wife Gail, and three other witnesses contradicted their accounts.

Two witnesses, Anne Colvin and Helen Allison, also told the jury they had seen Mr Sheridan having group sex with a prostitute and a footballer in a hotel in 2002.

Martha Rafferty, the women's solicitor, told The Herald: "My clients are pleased to hear that the Crown Office have given the go-ahead for further, more detailed inquiries into possible perjury during the trial. They anticipate that they will be interviewed by Lothian and Borders Police in the near future."

A Crown Office spokesman said: "It is not possible for us to speculate on possible lines of inquiry or timescales."

In the wake of the trial, Mr Sheridan split from the Scottish Socialists to form Solidarity.

A spokesman for the Scottish Socialists declined to comment on the latest developments, but added: "We have kept abreast of the police inquiry and have co-operated with it in whatever way we have been asked."

A Solidarity spokesman said he was not aware of any change. Mr Sheridan could not be contacted last night.