Interpol will be sent a request for an international search and arrest warrant for Sir Mark Thatcher today over his alleged part in an attempt to overthrow the government of Equatorial Guinea.

Simon Mann, 55, the former SAS officer who has confessed his role in an abortive coup in the tiny, oil-rich West African state in 2004, is gambling on reducing his prison sentence by helping the country’s legal authorities in their attempts to bring Sir Mark to trial over “the Wonga Coup”.

Mann is “cooperating fully” with prosecutors in Equatorial Guinea, according to the country’s attorney-gen-eral, José Oló Obono. Mr Obono showed The Times documents from a bulging file of evidence yesterday.

This constitutes his case against Sir Mark, the son of Baroness Thatcher. If found guilty, Sir Mark could face 30 years inside Equatorial Guinea’s notorious Black Beach prison.

The request to Interpol for an international search and arrest warrant for Sir Mark – whose whereabouts are currently unclear – also names five others accused of being the master-minds of the coup attempt.

The operation was foiled after South African intelligence blew the coup plans and Mann was arrested, with a planeload of mercenaries, while picking up fuel and weapons at Zimbabwe’s Harare airport en route to Mal-abo, capital of this former Spanish colony.

Since his extradition to Equatorial Guinea from Zimbabwe on January 30, Mann has spent days giving a detailed description of the planning behind the coup, in which he claims to have been “manager” for Eli Calil, a British businessman of Lebanese-Nigerian origin whom he accuses of being the coup attempt’s instigator.

Mr Calil has denied involvement in the plot categorically.

But Mr Obono said he was unhappy with the legal bargain that South Africa struck in January 2005 with Sir Mark, under which he admitted unwittingly abetting the coup by financing transport. In return he received a fine of three million rand – £266,000 – and a four-year suspended jail term but was allowed to leave South Africa. “It wasn’t a good decision by South Africa. It does not seem right to us that after his confession he paid a fine and they let him go,” Mr Obono said. “We are not going to let this drop. We will pursue him wherever he is.

“We must examine his complicity but we already know – there are signed documents to prove it – that he paid for aircraft,” he alleged.

The basis for the attempt is new information supplied by Mann. The Equitoreal Guinean attorney-general added that Mann would not have to appear as a prosecution witness if Sir Mark were to face a trial, but would rely on documents.

He confirmed that Mann’s strategy was to give a full account of the coup attempt.

“Mann is going to be tried, he will be condemned. But a person who cooperates, who doesn’t make things difficult . . . in those circumstances that could reduce his sentence.”

The trial had been expected to begin within days but Mr Obono said it was still possible that others might yet be in the dock alongside Mann.

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