Mark Thatcher was a leader of a 2004 coup plot in Equatorial Guinea that was backed by Spain and South Africa, the British mercenary Simon Mann told a court today.
Simon Mann, an Eton-educated former special forces officer, was giving testimony at his trial in Malabo about his role in the failed plot to overthrow President Teodoro Obiang Nguema Mbasogo in the small, oil-rich West African nation.
Mann said Margaret Thatcher's son, who has denied knowing about the conspiracy, "was not just an investor".
"He came on board completely and became part of the management team," Mann said, adding that a London-based millionaire, Eli Calil, was "the boss" of the plot.
Mann also said the governments of Spain and South Africa knew about the conspiracy and approved of it going ahead.
Spain's Foreign Ministry quickly denied the allegation.
Mann was arrested in Zimbabwe in March 2004 with 70 mercenaries en route to Equatorial Guinea, a former Spanish colony which Obiang has ruled since 1979.
On the first day of Mann's trial on Tuesday, the prosecution asked the court to jail him for nearly 32 years for his role in the coup plot, which he has admitted being part of. His defence lawyer said he was an "instrument", but not a ringleader.
Mann, 55, testified on Wednesday that by January 2004, the coup plot had become "like an official operation because the Spanish government and the South African government were both giving the green light".
"Their involvement was clandestine and they will never admit it," he told the court.
"We totally deny what Mr Mann says. We did not give the green light to any of this," a Spanish Foreign Ministry spokesman said in Madrid.
Equatorial Guinea's Public Prosecutor Jose Olo Obono told Reuters the next step for the West African state's authorities would be to seek the extradition of Thatcher and Calil.
"I am very, very sorry for what I have done. I am also very happy that we failed ... I think that the people that were seriously involved in this and have not faced justice, they should do so," Mann told the court
Yesterday, the prosecutor asked for a cumulative jail term of 31 years and eight months for Mann on charges of crimes against the head of state, crimes against the government and crimes against the peace and independence of the state.
In today's testimony, Mann said the plotters had rushed to try to carry out the coup before March 14, 2004, the date of a general election in Spain which they feared would unseat the centre-right government and deprive them of the Spanish diplomatic backing he said they had been promised.
"Everything was in ... a big hurry, because we had this date of March 14, the Spanish election, which was coming closer and closer," Mann said.
Spain's centre-right government led by Jose Maria Aznar was defeated in the 2004 election by the Socialists.
Mann said Calil had told him the coup plotters had been promised immediate diplomatic recognition by Aznar's administration if they succeeded in replacing Obiang with Equatorial Guinea opposition figure Severo Moto.
Spain had also promised to send a contingent of Civil Guards, according to Calil, Mann said.
He added that an intelligence contact, who liaised with South Africa's intelligence services, had asked him to provide Moto's telephone number so that South African President Thabo Mbeki could call Moto if the coup succeeded.
Calil, who made his fortune in Nigeria's oil sector, denies any involvement in the Equatorial Guinea coup plot.
The court heard on Tuesday Mark Thatcher was accused of paying the equivalent of $300,000 to be used mainly to purchase a helicopter to transport Moto from exile in Spain to Malabo, once Obiang was overthrown.
The prosecution said Calil invested some $2 million in the coup attempt and was its main financier.
Thatcher, who was arrested in 2004 by South African police on suspicion of bankrolling the plot, said he thought he was financing the helicopter for an air ambulance service.
He was released after pleading guilty to breaking South Africa's anti-mercenary laws and he paid a fine.
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