A dossier compiled by the government on Saddam Hussein's weapons of mass destruction was rewritten to make it "sexier", a senior British official has told the BBC.
UN inspectors failed to find weapons of mass destruction
The claim - hotly denied by Downing Street - came as Prime Minister Tony Blair became the first Western leader to visit post-conflict Iraq.
Published last September, the dossier warned that Saddam Hussein had the capacity to activate his biological and chemical weapons in just 45 minutes.
But the US defence secretary, Donald Rumsfeld, has suggested that the weapons might have been destroyed before the fighting began.
The intelligence official told the BBC the dossier had been "transformed" a week before it was published on the orders of Downing Street.
He said: "The classic example was the statement that weapons of mass
destruction were ready for use within 45 minutes.
"That information was not in the original draft. It was included in the
dossier against our wishes because it wasn't reliable.
"Most things in the dossier were double source but that was single source and
we believe that the source was wrong."
He said "most people in intelligence" were unhappy about the changes because they "didn't reflect the considered view they were putting
But the official said he was convinced that Iraq had programme to produce weapons of mass destruction, and felt it was 30% likely there was a biological weapons programme.
He said some evidence had been "downplayed" by chief UN weapons inspector Hans Blix.
But Iraqi scientists captured during the war had not provided much information as yet, he added.
Responding to the BBC report, Defence Minister Adam Ingram rejected suggestions that the US-led coalition had effectively gone to war on a false pretext.
He said the allegation that Downing Street had demanded changes to the dossier was untrue.
He told BBC Radio 4's Today programme: "The war was fought on the basis of all of the allegations, much of which was
substantiated, not just by a security document produced by our security
services, not concocted by Number 10 or under pressure from Number 10 to produce
it in a particular way...
"[It came from] their best knowledge and their best assessment of what they could declare into the public domain, based upon the knowledge of what
was out there.
"The whole world knew what Saddam Hussein was up to in terms of the weapons of mass destruction and that's why we prosecuted the war and that's why we were right."
This was echoed by senior sources inside the British intelligence community, who told the BBC on Thursday night that the heads of every agency that contributed to the dossier were satisfied with its contents.
They were nervous, however, that this was the first time secret material had been used in this way to support the government's case so publicly, the sources added.
Mr Ingram accepted that the suggestion that Saddam had weapons which could be used within 45 minutes was based on a single source.
But he said the "jigsaw was beginning to come into place" as the search for weapons goes on.
Mr Blair has said he is still absolutely sure that weapons of mass destruction will be found.
"Rather than speculating, let's just wait until we get the full report back from our people
who are interviewing the Iraqi scientists," he said on Wednesday.
Downing Street said: "Not one word of the dossier was not
entirely the work of the intelligence agencies."
The BBC report said the Parliamentary Intelligence and Security Committee is to conduct an inquiry into the UK Government's claims about Saddam Hussein's regime.
Liberal Democrat foreign affairs spokesman Menzies Campbell said the report added weight to rumours that the intelligence services were unhappy about the way their evidence was being used.
Labour MP Tam Dalyell, who opposed the war with Iraq, called for a Commons statement on the claims.
The new questions over the dossier came as CBS reported that the bunker that the US attacked in the hope of killing Saddam Hussein at the beginning of the war never existed.
The American network quoted US Army Tim Madere, who is in charge of inspecting key sites in Baghdad, as saying there was no trace of a bunker or of any bodies at the Dora Farms.