Sir Mark Thatcher said today that he was being "destroyed" by charges that he helped finance a failed coup in Equatorial Guinea.

The son of the former Prime Minister Baroness Thatcher said that his business would be ruined and he felt like a "corpse".

The melancholy interview, in Vanity Fair magazine, was published on the same day that the 51-year-old businessman learnt that he had lost his appeal not be questioned in open court by prosecutors from the oil-rich West African country.

A judge in South Africa's High Court ruled today that Sir Mark will have to answer about 50 questions posed by Equatorial Guinea's prosecutors. He was also ordered to pay costs.

The questioning will take place on Friday, unless he manages to delay the hearing with an appeal to the Supreme Court.

Sir Mark is separately facing criminal charges in South Africa of bankrolling the failed coup. He was arrested at his Cape Town home in August and charged in South Africa with violating its anti-mercenary law. He is scheduled to appear in court in Cape Town tomorrow, when a date for his trial will be fixed.

If he is convicted, he could be jailed in South Africa, or fined and extradited to Equatorial Guinea to face further charges in that country. He is currently barred from leaving South Africa.

Sir Mark told the magazine that his reputation was destroyed. "Thank God my father is not alive to see this," he said. "I will never be able to do business again. Who will deal with me?" There were 18,500 references to the case in the media on the day that he was arrested, he said.

He was "furious" that he was confined to South Africa, facing the courts. His telephone calls and e-mails are monitored by the authorities. He added that he felt "like a corpse that's going down the Colorado River and there is nothing I can do about it".

Sir Mark said he still speaks to his mother twice a week by telephone, but refused to disclose what they talk about. The interview is published in the next issue of Vanity Fair, which on sale from December 3.

Earlier today, Sir Mark's lawyers argued before Judge Dion van Zyl in the High Court in Cape Town that it was wrong to ask Sir Mark incriminating questions in open court when he was facing trial on related charges.

They also argued that his questioning could lead to the conviction and consequent execution of Nick Du Toit, the South African mercenary on trial in Equatorial Guinea for his alleged role in the coup attempt.

But the judge said: "It is pure speculation as to whether or not the accused in that trial may be convicted and sentenced."

Judge van Zyl ruled that the South African Government had not infringed Sir Mark's right to a fair trial or his right to silence to avoid self-incrimination. In a 45-page judgment, he said there was nothing "irrational or unconstitutional" in the way the subpoena was considered or approved.

There were unconfirmed reports from Equatorial Guinea last week that prosecutors there had charged Sir Mark in connection with the alleged coup plot. The country has already said it will seek his extradition from South Africa.

Jose Olo Obono, the Equatorial Guinea attorney general, said that Sir Mark was accused of helping to finance the coup attempt and he was added to a list of people believed to be behind the coup attempt..

Sir Mark's lawyer Alan Bruce-Brand said later that it was too soon to say what the next step would be. Earlier, he said if Sir Mark's court bid failed, he would probably answer the questions from Equatorial Guinea authorities on Friday.

Outside court, Sir Mark said: "The court has reaffirmed my right to silence."

Jonathan Clayton, Times correspondent in South Africa, said: "This is a very bad result for Sir Mark. We do not know if he will now appeal against this ruling.

"If the questioning goes ahead and Sir Mark says anything in open court that could be used by the prosecution then no doubt the South African prosecutors will use it. But he will probably be advised by his lawyer not to reply."

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