Simon Mann, the British mercenary freed from a West African jail, could become the star prosecution witness in a future trial of Sir Mark Thatcher and other alleged plotters in the failed Equatorial Guinea coup attempt.

Despite Mann’s claim that Sir Mark was a “main financier” of the plot to overthrow the Government of the oil-rich state in 2004, the son of the former Prime Minister denies any part in it.

However, Mann’s release and return to Britain makes both criminal and civil actions possible.

A civil action by Equatorial Guinea for damages against Mann and the other alleged plotters collapsed last year when the law lords refused to hear it because the authorities in the country were refusing to give Mann access to lawyers or consular advice. That action could now resume.

A criminal trial may also go ahead. Scotland Yard anti-terrorist detectives are preparing to interview Mann, who has said he is willing to co-operate.

The police inquiry, Operation Antara, began in earnest last summer at the end of Mann’s trial in Equatorial Guinea and a small team from Scotland Yard visited the country on three occasions in July, September and October 2008, receiving files from the authorities there.

The Crown Prosecution Service declined to say what potential breaches of the law were being considered.

Sir Mark, the son of Baroness Thatcher, admitted to a South African court that he had unwittingly financed part of the plot, after paying for a helicopter which he said he believed was an air ambulance. After a plea bargain, he was fined £265,000 and was given a four-year suspended sentence. After Mann was arrested with other mercenaries at Harare airport in March 2004, he named Sir Mark as one of the main financiers “in charge of the operation”, and told the court that Ely Calil, a Lebanese-Nigerian tycoon, based in London, was “the boss” who masterminded the botched coup.

Both men have denied the claims.

Friends of Sir Mark said last night they were confident he would not have to stand trial. They said: “The police know where Mark Thatcher is, and they have made no approach to him.”

When asked about the possibility of Sir Mark facing trial, the source said: “It is really not going to happen.”

He had to leave South Africa in 2005, his home of ten years, where he had enjoyed a privileged lifestyle in Constantia, an exclusive district in Cape Town. He now lives most of the year in a high-security luxury compound in southern Spain.

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