Blair's office rejected the British Broadcasting Corp.'s report, which cited an intelligence source.
"Not one word of the dossier was not entirely the work of the intelligence agencies," it said in a statement.
An unidentified expert in Britain's intelligence network told the BBC the 50-page document contained unreliable information and was "transformed" on instructions from Blair's office in the week before its release last September, to make it "sexier."
"The classic example," the BBC quoted the intelligence officer as saying, "was the statement that weapons of mass destruction were ready for use (by Iraq) within 45 minutes."
In the dossier, Blair had warned that Iraqi President Saddam Hussein could activate a chemical and biological arsenal in that time -- a suggestion that became a pillar of Britain's rationale for going to war alongside the United States against Baghdad.
"That information was not contained in the original draft" that had been prepared for the prime minister, he said. "It was included in the dossier against our wishes because it wasn't reliable."
The claim came in the wake of U.S. Defense Secretary Donald Rumsfeld's comment that Saddam's regime might have destroyed its chemical and biological weapons before the fighting began and amid growing suggestions the coalition's war had been launched on the weapons pretext.
Defense Minister Adam Ingram told BBC Radio "the war was fought on the basis of all of the allegations, much of which was substantiated, not by just a security document produced by our security services, not concocted by Number 10 (Blair's office) or under pressure from Number 10 to produce it in a particular way."
The intelligence officer interviewed by the BBC conceded that "most things in the dossier were double sourced, but that (claim about Iraq's ability to launch weapons of mass destruction on 45 minutes' notice) was single source, and we believe that the source was wrong."
"Most people in intelligence weren't happy with the dossier because it didn't reflect the considered view they were putting forward," he said while claiming that a "transformation" had taken place under orders from Downing Street.
Blair, currently visiting Iraq to thank British forces for their role in the overthrow of Saddam's regime, said earlier that "rather than speculating (about the weapons of mass destruction), let's just wait until we get the full report back from our people who are interviewing the Iraqi scientists."
The BBC's intelligence source said it was "30 percent likely" Iraq did, indeed have a biological weapons program under way, and he suggested that U.N. weapons inspectors themselves may have understated some of the evidence.
"We think U.N. weapons inspector Hans Blix downplayed a couple of potentially interesting pieces of evidence," though "the weapons programs were small (and) sanctions did limit the programs." He did not elaborate.
As for interviewing Iraq's scientists, the intelligence officer said that so far, "We don't have a great deal more information yet than we had before. We have not got very much out of the detainees yet."
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