Tony Blair still believes "concrete evidence" of the products of Saddam Hussein's weapons programmes will be found, says Downing Street.
Downing Street says Blair's stance remains unchanged
The statement comes despite
senior Whitehall sources telling the BBC they no longer believe actual weapons of mass destruction are likely to turn up in Iraq.
The prime minister's official spokesman said that it was Mr Blair who was the authentic voice at the top of government.
"He is absolutely confident that we will find evidence
not only of weapons of mass destruction programmes but concrete evidence of the product of those
programmes as well."
BBC political editor Andrew Marr says "very senior sources" have virtually ruled out the possibility of finding weapons in Iraq.
They believe they did exist - but were hidden or destroyed by Saddam Hussein before the war.
Mr Blair's spokesman would not be drawn on whether actual weapons would be found.
But he said: "This is another BBC exclusive based on another anonymous source.
"The prime minister believes and has absolute confidence we will find material that had (UN chief weapons inspector) Hans Blix found it would have justified him going back to the Security Council for a further UN resolution."
Number 10 insisted Mr Blair had not shifted his language to talk about programmes, rather than weapons themselves.
Labour MP Brian Donohoe has suggested Mr Blair's job is at stake in the search for evidence.
He told BBC Radio Scotland: "If I don't see the evidence that's overwhelming in terms of the weapons of mass destruction having been there, I'm not going to call, but I do believe that the position of the prime minister in these circumstances is untenable and that he would have to resign."
Earlier, Robin Cook, who resigned as leader of the Commons on the eve of war, told BBC Radio 4's Today programme:
"Parliament voted for war because it was told that Saddam did have real weapons of mass destruction.
"Indeed what the prime minister said on the eve of the war was that the weapons posed a real and present danger, either because [Saddam] might use them or because he might pass them to terrorist groups."
Former prime minister John Major said an independent inquiry to clear up the row was in the government's interest.
He said he had backed the move to war because he had accepted the government's warnings but said "quite a few" of those alerts were now being questioned.
"We are in the middle of the war against terror and nobody knows what our troops may be asked to do next," he went on.
"It is essential that the word of government and the intelligence services is readily accepted by Parliament and the public."
Conservative leader Iain Duncan Smith said an independent judicial inquiry into the weapons evidence was needed "more than ever".
Donald Anderson, chairman of the Commons foreign affairs committee, said such an inquiry would take a long time and might not reveal anything new.
"What I am certain of is that at the relevant time our people did believe that there were chemical and biological weapons what could have been used against our troops in the field," he said.
The latest twist in the weapons row came as the US Defence Secretary
Donald Rumsfeld admitted the US had obtained no fresh intelligence about weapons of mass destruction in Iraq before going to war.
But it saw existing information in a new light after 11 September, he told the Senate Armed Services Committee.
And he said weapons would eventually be found in Iraq.
Andrew Marr stressed that government and Whitehall sources were "still sure" Iraq did have major weapons of mass destruction programmes before the war.
"The assumption is that Saddam Hussein for whatever reason destroyed them or hid them beyond finding before the war started," he told Today.
"And there's no doubt also in their minds that they will turn up interviews with scientists, paper documentation and so on.
"But nobody's been killed by paper documentation ever, and it does change the nature of things."
It was still possible the Iraqi Survey Group - a team of experts tasked with looking for evidence of weapons - might turn up some weapons, he said.
"But they're clearly now preparing the ground for that not to be the case."
Lib Dem spokesman Menzies Campbell said the news undermined the legal advice given to the government about going to war.