SIR MARK THATCHER will escape the prospect of a 15-year jail term today by striking a deal with the South African authorities over his alleged involvement in a coup to topple the dictator of oil-rich Equatorial Guinea.

Baroness Thatcher’s son will pay a £275,000 fine or accept a five-year suspended jail sentence by admitting in court that he financed the lease of an air ambulance that he suspected was to be used for mercenary purposes.

Sir Mark’s humiliating admission of guilt comes after months of protesting his innocence. His plea bargain over the air ambulance will head off the threat of extradition to Equatorial Guinea, where the death penalty still exists.

Sir Mark planned to leave Cape Town immediately for Dallas, Texas, to be reunited with his with Diane and their children Michael, 15, and Amanda, 11. They have lived there since his arrest in August.

But there were questions over whether the US immigration authorities would admit him as he has a criminal record. A spokesman for the US Embassy said: “The US does not wish to have living inside its borders undesirable aliens, which includes people with criminal records. These people do not form model future citizens.”

However, the fact that Sir Mark’s wife is an American citizen would improve his chances. “She may be able to petition from the US for permission for her husband to enter the country. There's always an appeal process,” the spokesman said.

“Certainly there are circumstances when individuals who have been convicted of serious crimes are permitted to travel to the United States. Each case is judged on its merits.”

Sir Mark’s plea to a lesser charge of contravening section two of South Africa's tough anti-mercenary laws was made on the advice of his lawyers who warned him that he faced years of legal proceedings which would mean he could never leave South Africa.

Conspirators in the deal, including Crause Steyl, a veteran South African mercenary, had told investigators how the details of the coup were discussed at a Christmas party, at which Lady Thatcher was present but not party to the discussion, at Sir Mark’s home in the wealthy suburb of Constantia in 2003. Lady Thatcher, who spent last Christmas with her son and was kept informed of the legal negotiations, is deeply distressed that he leaves South Africa with a criminal record. A spokesman for her said: “She is very relieved that matters have now been settled and that the worry of these last few months is now over.”

Sir Mark, who has lived in South Africa since 1995, is also said to be relieved that a deal has been done. A source close to him said: “The charges he has pleaded to do not include anything connected with an attempted coup. He financed the hiring of the helicopter, to be used as an air ambulance, began to suspect it would be used for another purpose and he never informed authorities. That is the case against him. He agreed to do the deal because this would have gone on for years and he would never have seen his family.”

A legal source on the case said: “Basically, Thatcher will be admitting he was involved . . . He will be saying he did not know what he was involved in financing, but should have known. The price is a criminal record, but it was either that or facing years of trials and appeals, and higher legal fees than even this enormous fine.

The deal does not prevent the authorities in Equatorial Guinea seeking his extradition or President Obiang suing him in the High Court in London.

Despite his protestations of innocence the case against Sir Mark appeared to strengthen as one after another of his coaccused turned state evidence. He was accused of financing a helicopter that would have flown the exiled opposition leader Severo Moto into Equatorial Guinea hours after the overthrow of President Obiang.

Steyl, a pilot and businessman, negotiated a deal with the authorities before Christmas in which he agreed to testify in court that Sir Mark had paid him £145,000 to fly the helicopter into Equatorial Guinea. He said at several meetings that Sir Mark, who maintained that it was a simple lease deal, was made aware of what the helicopter would be used for.

Rumours of a coup plot against the corrupt government of Equatorial Guinea had been swirling around West Africa for months before it burst into the open last March when Sir Mark’s friend and neighbour Simon Mann and 66 former members of South Africa's armed forces were arrested in Zimbabwe. Their plane had stopped off to take on an arms consignment before flying on to Equatorial Guinea.

At the same time, a South African mercenary, Nick du Toit, and a group of about a dozen Armenians and South Africans were arrested in Equatorial Guinea. Du Toit is serving 34 years’ jail in Malabo.

MAIN PLAYERS

SIMON MANN: Former SAS officer and Old Etonian friend of Sir Mark arrested in Harare with a planeload of 66 mercenaries en route to Equatorial Guinea. Sentenced to seven years’ jail

NICK DU TOIT: South African mercenary arrested with co-conspirators in Equatorial Guinea. Now serving 34 years

CRAUSE STEYL: Ran Triple A Aviation, which was to ferry Mann’s mercenaries and the Equatorial Guinea opposition leader, Severo Moto, into Malabo. Agreed a plea bargain

ELI CALIL: London-based Lebanese billionaire accused of being the key financier. Denies knowledge of the coup

JEFFREY ARCHER: A cheque for $134,980 from J. H. Archer was paid to a firm owned by Mann, which Equatorial Guinea claims was paying the mercenaries. Archer denies knowledge of the coup

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