The British government investigated the possibility that British firms were involved in a plot to overthrow the president of Equatorial Guinea several weeks before last March's attempted putsch.
British Foreign Office officials and diplomats in the region took the coup threat so seriously that they rewrote contingency plans to evacuate British nationals from the oil-rich central African state.
However, the government failed to alert its Equatorial Guinean counterpart of the threat.
Last August, the Foreign Office issued a "categorical" denial that it had any prior knowledge of the coup, but has now confirmed that officials received information at the end of January about the threat as a result of "confidential diplomatic exchanges."
The admissions by Jack Straw, the UK's Foreign Secretary, revealed in a parliamentary answer, will raise questions about why the Foreign Office did not act to avert the coup, as is its duty under international law.
On 7 March, a group of mercenaries allegedly linked to Sir Mark Thatcher, the son of the former Prime Minister, was arrested in Zimbabwe on charges of planning a coup against President Teodoro Obiang.
At the time, Zimbabwe's President Robert Mugabe accused Britain, the US and Spain of being behind the coup in an attempt to gain control of the country's oil interests.
Last week, the London-based Observer newspaper revealed that Straw had been informed of the alleged coup in late January. Now it has emerged that not only did the government know about the plot, but that it also took steps to establish whether British firms and nationals were involved. Straw has refused to reveal who provided the information and what that information was. He said: "We do not provide details of confidential diplomatic exchanges."
Sources claim the South African intelligence service had detailed information on those behind the coup that intended to replace Obiang with Severo Moto. This is alleged to include information on one of the ringleaders, Simon Mann, an Old Etonian and former SAS officer.
In a written parliamentary answer to the opposition Conservative party foreign affairs spokesman Michael Ancram, Straw said the Foreign Office was aware of reports circulating in the Spanish media in January.
"At the same time, similar allegations were contained in confidential information received by the government," he added. "We were sceptical about the reports, as there had been a number of coup rumors in the media, including October 2003."
"Insofar as we could, we attempted to establish whether there was any more truth to this particular allegation. We took action to try to establish whether any UK companies were involved and to underline our opposition to involvement by any company in such activities," he said.
Straw added that officials could find no "definitive" evidence of the plot and therefore did not warn the government of Equatorial Guinea.
However, the Foreign Secretary confirmed that the Foreign Office did review and update its civil contingency plans. Britain does not have an embassy in Equatorial Guinea, but its interests are represented via Cameroon.
Ancram claims that Straw's parliamentary replies "raise more questions than they answer." He has demanded to know why, if the Foreign Office did not believe the allegations of a potential coup, it needed to change the civil contingency plans.
Ancram's office has also questioned how thorough the Foreign Office investigations were, given that several of the alleged plotters were either British citizens or lived in Britain.
Mann's company, Logo Logistics, was registered in Guernsey and he was well known to the circle of former British military men who run private security firms.
In addition, Mann is alleged to have meetings in London, with millionaire oil trader Ely Calil. Both men deny involvement in the plot. On Friday, Thatcher will face trial in South Africa.