A British mercenary awaiting trial in Equatorial Guinea for leading a failed 2004 coup has said the son of former British Prime Minister Margaret Thatcher was involved in the plot, the public prosecutor said on Sunday.

Jose Olo said former British special forces officer Simon Mann had testified that Mark Thatcher knew all about the scheme to overthrow President Teodoro Obiang Nguema, who has ruled the oil-rich West African country since 1979.

Thatcher has in the past admitted chartering a helicopter for Mann, who was arrested in 2004 in Zimbabwe with 70 mercenaries en route for Equatorial Guinea, but he has denied any knowledge of the planned coup.

Olo said authorities in Equatorial Guinea -- sub-Saharan Africa's third-largest oil producer -- were still taking testimony from Mann ahead of a trial expected to start in May.

Mann admitted in an television interview broadcast in Britain this month that he plotted to oust Obiang.

"He has testified that Thatcher knew all about the operation," Olo told Reuters. "If we can gather enough evidence we will start a case against (Thatcher). ... We have still not taken a decision."

Media reports on Saturday said Equatorial Guinea had issued an international arrest warrant for Thatcher, but Olo said no such official steps had been taken.

A spokeswoman for international police agency Interpol said Equatorial Guinea had not sought its help in arresting Thatcher, which generally happens if a country wants to enforce an arrest warrant abroad.

In 2004, Equatorial Guinea issued international arrest warrants for those it suspected of involvement. Thatcher was detained by South African police that year on suspicion of bankrolling the scheme but struck a plea bargain to avoid prosecution and paid a large fine.

Thatcher has shrugged off any suggestion Equatorial Guinea may have him arrested saying he felt "pretty relaxed about it".

"As far as I'm concerned the issue has already been dealt with," he told London's Daily Telegraph newspaper.

"I've been charged and tried in a court in South Africa on exactly those charges so I don't see what more they can do. The term 'double jeopardy' springs to mind."

Other suspects in the coup plot included London-based businessman Eli Calil and Severo Moto, who leads a self-proclaimed Equatorial Guinea government-in-exile in Spain.

Mann was jailed for four years by a Zimbabwe court in 2004 for buying weapons without a licence -- which the prosecution said were intended for the coup.

He was extradited in February by President Robert Mugabe's government, which received an oil supply deal from Equatorial Guinea last year.

Mann, who is being held in Malabo's notorious Black Beach prison, faces a possible life sentence.

The 55-year-old Mann -- heir to a brewing fortune who attended Britain's exclusive school Eton -- helped found two security firms which became bywords for mercenary activity across Africa in the 1990s.

Obiang overthrew and killed his uncle Francisco Macias Nguema -- one of Africa's most brutal dictators -- in a 1979 coup. His government is accused of widespread human rights abuses.

Meanwhile a British press report said Thatcher had remarried in a secret ceremony in Gibraltar.