Its ruler, President Teodoro Obiang, was believed to be dying of cancer, and valuable oil concessions were hoped to be up for grabs. Much of the country's mushrooming oil industry is controlled by US companies.

Sir Mark is in limbo, staying at his mother's house in London while he attempts to renew his visa to gain entry to the US. The US authorities are deciding whether to grant him admission, despite his having a criminal conviction.

The former prime minister's son fled South Africa following his conviction and £270,000 fine there for financing a helicopter gunship to be used in the coup. In a plea bargain, Sir Mark admitted investing in the mercenaries' scheme, despite realising the helicopter "might" be used for mercenary activity. He and his friends have tried to present his role as unwitting and peripheral.

But the new evidence appears to place him at the centre of events. Phone records which the Guardian has seen show him placing two international calls from his home in South Africa to the mobile phone of Eli Calil, then based in his London mansion in Chelsea.

The calls were made within half an hour on February 2 last year, when planning for the coup was at its height. A fortnight earlier, Sir Mark had invested $275,000 during meetings in South Africa. Other alleged plotters had travelled to Spain to brief their candidate for president, the exiled African politician Severo Moto. In a third key leg of the alleged plan, a British businessman, Greg Wales, went to the US and hired a lobbyist who had influence in Washington.

Joseph Sala, who now runs the lobbying firm the ANN Group, says in tonight's programme: "The arrangement that we struck with Wales and the friends of Moto [unidentified] in February was that we would be paid $40,000 to put together a four-day programme for Moto in Washington, access to the Congress, think-tanks, media". He tells the programme, Thatcher and the Coup that Failed, that "the assumption in Washington would be that Calil wanted access to Equatorial Guinea's oil and that he, Calil, was prepared to do whatever was necessary to bring Moto to power on the assumption that Moto would return the favour.

"It's callous, it's crude, but it's the way of the world."

Mr Sala says it was only when he visited state department officials on Mr Moto's behalf that he discovered the true financial backer was Eli Calil. He says the plotters were not honest with him, and concealed their intention to use mercenaries to overthrow the regime. Lord Bell, the public relations man who has been acting as spokesman for Sir Mark, told the Guardian he had no comment on the allegations about Sir Mark's phone calls. Mr Calil and Mr Wales are both still at liberty and deny involvement in the coup attempt.