The allegation follows a Downing Street briefing by a senior official to newspapers on Friday which claimed US forces had discovered a biological weapons laboratory in a cave in eastern Afghanistan after fighting near the city of Gardez this month.

A 'senior Whitehall source' gave detailed claims of how American soldiers had found the cave following heavy fighting for al-Qaeda positions around the village of Shah-e-Kot.

One report quoted the source as saying: 'We know from documents found in Kabul and the lab in the cave that Osama bin Laden has acquired a chemical and biological weapons capability.'

The newspapers reported that the find was one of the main reasons the Government had decided to send the Marines to Afghanistan to fight al-Qaeda and Taliban forces. The claim, carried by a number of newspapers yesterday, was denied emphatically last night by Pentagon and State Department sources.

A White House spokesman, drawn into the row, said 'no evidence' had yet been uncovered in Afghanistan that Al Qaeda had succeeded in producing anthrax or other biological or chemical agents.

A Pentagon official told The Observer there was no intelligence to support claims from London that al-Qaeda was developing biological weapons in the Shah-e-Kot area. 'I don't know what they're saying in London but we have received no specific intelligence on that kind of development or capability in the Shah-e-Kot valley region - I mean a chemical or biological weapons facility,' said an official in the Army department in Washington.

The US rebuttal came as Opposition spokesmen demanded that Defence Secretary Geoff Hoon address the House of Commons to 'clarify' the claims, amid growing backbench unrest about the way in which the decision to send the marines was made.

The first of them are due to arrive in Kabul in the next few days to join US combat troops already fighting on the ground, amid concern among MPs about the 'open ended' nature of their mission.

Liberal Democrat foreign affairs spokesman Menzies Campbell, who called for Hoon to make a statement, said: 'The House will feel, with some justification, that this claim was leaked to the media to justify the deployment after the event.

'There are too many unanswered questions about the military justification for this deployment and growing unease. Mr Hoon owes the House a clarification."

The Tories demanded that Downing Street stick strictly to the truth in its efforts to promote the military campaign. 'Spinning doesn't work for the NHS, so why do they think it is going to work for the war on terrorism?' said Bernard Jenkin, Shadow Defence Secretary.

Doubts about the story surfaced almost immediately after it was published, as US officials first expressed bafflement and then denied any such lab had been found. Some speculating to the New York Times that the story might have been planted to justify the deployment of the marines. British intelligence, Ministry of Defence and Foreign Office sources denied any knowledge of the lab.

The only evidence of a biological weapons laboratory was the discovery last December of an abandoned, half-finished building containing medical equipment, near the Taliban's former power base of Kandahar in southern Afghanistan. This had been reported previously.

The Observer has established that the source of the claims was an off-the-record briefing by Tony Blair's senior foreign policy adviser, David Manning.

A Downing Street spokesman said it 'stuck by the thrust of the story' - that it had evidence al-Qaeda was 'interested' in acquiring such weapons. But Manning had 'not actually told' reporters a cave lab had been discovered.