American officials lied to British ministers over the use of "internationally reviled" napalm-type firebombs in Iraq.
Yesterday's disclosure led to calls by MPs for a full statement to the Commons and opened ministers to allegations that they held back the facts until after the general election.
Despite persistent rumours of injuries among Iraqis consistent with the use of incendiary weapons such as napalm, Adam Ingram, the Defence minister, assured Labour MPs in January that US forces had not used a new generation of incendiary weapons, codenamed MK77, in Iraq.
But Mr Ingram admitted to the Labour MP Harry Cohen in a private letter obtained by The Independent that he had inadvertently misled Parliament because he had been misinformed by the US. "The US confirmed to my officials that they had not used MK77s in Iraq at any time and this was the basis of my response to you," he told Mr Cohen. "I regret to say that I have since discovered that this is not the case and must now correct the position."
Mr Ingram said 30 MK77 firebombs were used by the 1st Marine Expeditionary Force in the invasion of Iraq between 31 March and 2 April 2003. They were used against military targets "away from civilian targets", he said. This avoids breaching the 1980 Convention on Certain Conventional Weapons (CCW), which permits their use only against military targets.
Britain, which has no stockpiles of the weapons, ratified the convention, but the US did not.
The confirmation that US officials misled British ministers led to new questions last night about the value of the latest assurances by the US. Mr Cohen said there were rumours that the firebombs were used in the US assault on the insurgent stronghold in Fallujah last year, claims denied by the US. He is tabling more questions seeking assurances that the weapons were not used against civilians.
Mr Ingram did not explain why the US officials had misled him, but the US and British governments were accused of a cover-up. The Iraq Analysis Group, which campaigned against the war, said the US authorities only admitted the use of the weapons after the evidence from reporters had become irrefutable.
Mike Lewis, a spokesman for the group, said: "The US has used internationally reviled weapons that the UK refuses to use, and has then apparently lied to UK officials, showing how little weight the UK carries in influencing American policy."
He added: "Evidence that Mr Ingram had given false information to Parliament was publicly available months ago. He has waited until after the election to admit to it - a clear sign of the Government's embarrassment that they are doing nothing to restrain their own coalition partner in Iraq."
The US State Department website admitted in the run-up to the election that US forces had used MK77s in Iraq. Protests were made by MPs, but it was only this week that Mr Ingram confirmed the reports were true.
Mike Moore, the Liberal Democrat defence spokes-man, said: "It is very serious that this type of weapon was used in Iraq, but this shows the US has not been completely open with the UK. We are supposed to have a special relationship.
"It has also taken two months for the minister to clear this up. This is welcome candour, but it will raise fresh questions about how open the Government wished to be... before the election."
The MK77 bombs, an evolution of the napalm used in Vietnam and Korea, carry kerosene-based jet fuel and polystyrene so that, like napalm, the gel sticks to structures and to its victims. The bombs lack stabilising fins, making them far from precise.