The Foreign Office is playing down reports it had full details of a coup plot in Equatorial Guinea two months before it was foiled.
Simon Mann was convicted earlier this year
Foreign Secretary Jack Straw was told of the plans in January, according to a report in The Observer newspaper.
But he did not share the reports with African authorities, apparently believing it added nothing to rumours already circulating in the media.
The Foreign Office said it "acted promptly" on "relevant information".
Conservative party foreign affairs spokesman Michael Ancram has demanded more details of what the government knew, when it received information, and from whom.
The coup was foiled in March when a team of mercenaries were arrested in Zimbabwe, as they prepared to pick up a consignment of weapons.
A number of people, including Briton Simon Mann, have been convicted and sentenced on weapons charges. Mann denies he was on his way to stage a coup.
In South Africa Sir Mark Thatcher, son of former British Prime Minister Margaret, is awaiting trial for allegedly helping to bankroll the coup - a charge he denies.
The Observer said Mr Straw and his junior minister for Africa, Chris Mullin, had been told of the coup plot on 30 January, but failed to warn the government of the small, oil-rich west African state.
It said two "highly detailed" reports had been sent, in December 2003 and January this year, from Johann Smith, a former commander in the South African Defence Forces, to two senior British intelligence officers.
It said the documents featured the names of many South African mercenaries who have since been sentenced for their role in the plot.
The January report stated the coup would be attempted "in mid-March 2004".
The author of the report added: "Knowing the individuals as well as I do, this timeline is very realistic and will provide for ample time to plan, mobilise, equip and deploy the force."
The Foreign Office told the paper: "We do not comment on intelligence issues.
Jack Straw insisted the Foreign Office acted properly
"But ministers and officials (in the ministry) acted promptly on receipt of relevant information."
In a written answer to a question from shadow foreign secretary Michael Ancram, Mr Straw claimed first reports of the coup had circulated in Spanish newspaper El Mundo in late January.
He said the Foreign Office did not share details of the reports because of regulations governing "confidential diplomatic exchanges".
Mr Straw added: "As we were not able to establish any definitive evidence which could add significantly to the reports which had already appeared in the media we took no further action with other African governments. But we did review and
update our civil contingency plan."
Mr Ancram told BBC Radio 4's Today programme on Monday all the information had not yet been made available to the public.
"I just want to get to the bottom of this. I think it is one of these stories where we are not being told the whole truth, and I think we ought to be," he said.
Mr Ancram has also questioned the government on reports former Labour MP Peter Mandelson had contact with Jack Straw's office in relation to the coup, and to clarify suggestions there were discussions between the UK, French and Spanish government over the plot.
Proceedings against Sir Mark over allegations he helped finance the coup plot have been postponed until April next year.
Sir Mark was 'furious' at being barred from leaving South Africa.
Sir Mark, 51, had been expected to enter a plea in a South African court on Wednesday, but prosecutors wanted more time.
The businessman had his bail conditions extended, meaning he is confined to the Cape Town area for at least the next five months.
Equatorial Guinea, Africa's third biggest oil producer, says it intends to extradite him to face similar charges as 14 other suspected foreign mercenaries currently on trial there.