The spectacular collapse last March of the coup in tiny Equatorial Guinea, sub-Saharan Africa's third-largest oil producer and among its most rigid dictatorships, has roiled South Africa for months. Most of the accused participants are either South Africans or holders of South African passports.

Mr. Thatcher is a friend of Simon Mann, a veteran of British special military operations who is accused of organizing the coup attempt. Mr. Mann, who has run private military companies in South Africa and Britain, was among 70 suspected mercenaries arrested March 6 in Zimbabwe after their jet stopped in Harare on a flight from South Africa, apparently to pick up a cache of arms.

At the same time, the police in Malabo arrested 19 foreigners, many of them South Africans, and charged them with plotting a coup against the nation's strongman, Teodoro Obiang Nguema Mbasogo. One of the arrested men, a German, has since died in an Equatorial Guinea prison of what officials said was cerebral malaria.

Mr. Obiang, Equatorial Guinea's ruler since overthrowing and executing his uncle in 1979, recently told a French publication, Jeune Afrique/L'Intelligent, that ''certain elements also indicate that Thatcher and a former Thatcher cabinet minister whom I cannot name handled the financial planning of the coup.''

Mr. Thatcher told The Daily Telegraph in London last month that he knows Mr. Mann, but he declined further comment. The 89 suspected mercenaries have long denied that they were seeking to overthrow Mr. Obiang, saying they had been hired to provide security for gold-mining operations in the Congo Republic, southeast of Equatorial Guinea.

But in the Malabo trial, a South African arms dealer named Nick du Toit, identified as a key figure in the coup, admitted to the plot this week. He testified that Mr. Mann had offered him $1 million to provide invading mercenaries with vehicles, logistical support and information on the locations of Mr. Obiang and other key officials.

In testimony on Wednesday, wire services reported, Mr. du Toit stated that he was present in July 2003 at a meeting with Mr. Thatcher and Mr. Mann that Mr. Mann had arranged. But Mr. Thatcher was interested only in purchasing military helicopters for a mining operation in Sudan, Mr. du Toit testified.