The British government knew about the alleged plot to overthrow the president of Equatorial Guinea at least five weeks before a group of mercenaries was arrested in March for planning the coup.
In a dramatic admission, Foreign Secretary Jack Straw confirmed that the government had been "informed" of the alleged coup plot "in late January 2004."
On March 7 a group of mercenaries, led by a former SAS officer, Simon Mann, was arrested in Zimbabwe. They were charged with plotting a putsch.
Straw's disclosure is the latest twist in a remarkable tale that has dragged in several high-profile figures. In August, Sir Mark Thatcher, the son of the former British Prime Minister, was arrested in South Africa after being accused of helping to finance the coup to remove President Teodoro Obiang Nguema. He faces criminal charges that he broke the country's anti-mercenary laws. Thatcher denies any knowledge or involvement in the plot.
Straw's admission came in a parliamentary answer last week to a question tabled by the Conservatives' foreign affairs spokesman, Michael Ancram.
Until now, Cabinet ministers have denied any prior knowledge of the attempted African coup which would be illegal under international law.
In August it was reported that an individual intimately involved in the alleged plot against Obiang was claiming British officials had advanced knowledge of the plot. Foreign Office officials dismissed the claims, issuing a categorical denial that Britain had "prior knowledge of the alleged plot."
At the time of the March arrests, Zimbabwe's President Robert Mugabe accused Britain, the US and Spain of plotting a coup in the oil-rich West African state. The suggestion was that the coup was an attempt to gain control over Equatorial Guinea's new-found oil wealth that has turned the small country into Africa's third-biggest oil producer. The allegations have been strongly denied by foreign governments.
Yet the admission by Straw that the government had been informed of the coup plot several weeks in advance has raised questions about the role played by Britain. Senior opposition politicians are demanding to know who informed ministers and what they then did with the information received.
A source close to the government of Equatorial Guinea described Straw's admission as being "very surprising."
He said that Obiang would be seeking an immediate explanation from Straw as to why no warning was passed to the government of Equatorial Guinea, a country with which Britain has full diplomatic relations.
He added: "This is particularly surprising in view of the fact that a number of British citizens and residents of the UK appear to be central to the conspiracy to overthrow the government of Equatorial Guinea."
As well as Mann and Thatcher, other British-based individuals linked to the plot include Ely Calil, a Lebanese millionaire oil trader who lived in Chelsea.
Calil, who has temporarily moved to Lebanon, denies any involvement in bankrolling the coup, which allegedly aimed to replace Obiang with an exiled politician, Severo Moto.
Senior detectives at Scotland Yard are investigating claims by ministers from Equatorial Guinea that the plot was largely planned and financed in Britain.
"Jack Straw's reply raises very significant questions which require answers. Who informed the government, exactly when and what did ministers do with this information?" Ancram said.