A dossier including the claim that Iraq could launch weapons of mass destruction in 45 minutes was repeatedly returned to intelligence chiefs for changes, the BBC has learned.
Blair says the charges are totally untrue
A source close to British intelligence has told BBC diplomatic correspondent Barnaby Mason that Downing Street returned draft versions of the dossier to the Joint Intelligence Committee "six to eight times".
He said Prime Minister Tony Blair was involved in the process at one point.
Mr Blair has vigorously denied that the document was "sexed up" in order to garner support for war.
They have debased their own credibility by only issuing denials
The prime minister is under pressure over the way the government made the case for war in Iraq, with coalition forces yet to find weapons of mass destruction.
The chief UN weapons inspector Hans Blix returned to the UN Security Council on Thursday to present what may be his final report before his retirement.
He said Iraq had left "many unanswered questions" about its unconventional weapons, but this did not mean such dangerous arms still existed.
The first of 1,400 military experts have now arrived in Iraq to begin a new search for evidence of weapons of mass destruction.
Responding to Barnaby Mason's report, Mr Blair's office again said no pressure had been put on the intelligence services to change the document.
Mr Blair said on Wednesday he was confident the UN team, from the US, UK and Australia, would find signs of the nuclear, chemical or biological weapons.
"I have absolutely no doubt at all that they will find the clearest possible evidence of Iraq's weapons of mass destruction," he said.
But a former senior intelligence official in the US state department, Greg Thielman, said he thought evidence had been distorted - a charge also denied by the Bush administration.
Tory leader Iain Duncan Smith repeated his call for a full independent inquiry into whether intelligence documents on Iraq's weapons were changed on the orders of Downing Street to strengthen the case for military action.
He accused Commons leader John Reid of "debasing the credibility the government was standing on" by claiming "rogue elements" in the security services were briefing against the government.
"The government needs the British people to be able to believe, as we do, that they took decisions on the basis of intelligence that was right and that at the end of the day we can believe that intelligence," he told BBC Radio 4's Today programme.
"They have debased their own credibility by only issuing denials."
Former Labour chancellor Lord Healey has said Mr Blair should resign if he is wrong about weapons of mass destruction (WMD).
On Wednesday the Lib Dems, backed by the Tories, were defeated in a motion calling for a judicial inquiry into the matter, by 301 votes to 203.
Mr Blair again insisted it was "completely and totally untrue" that a dossier had been "sexed up".
But he said he would allow the all-party intelligence and security committee (ISC) to conduct an inquiry into the row.
Mr Blair said the disputed claim over Iraqi weapons strikes within 45 minutes was entirely the work of the JIC.
But another military source, said to be "intimately involved" in the compilation of the dossier, told BBC2's Newsnight programme he had been "uneasy" with the 45-minute claim.
The source, who the programme said could not be described as a "rogue
element", believed the emphasis placed on the claim had turned a possible capability into an imminent threat.
But he did not dispute the assertion that the intelligence services had put the claim into the dossier.
MPs on the influential foreign affairs select committee have already said they are to investigate the way the government presented intelligence information over Iraq's weapons.
Former foreign secretary Robin Cook said while he gave "two cheers" to the two investigations, he would have preferred "a more open and transparent inquiry", conducted by somebody from outside politics.
Commons leader Dr John Reid stirred up the row further on Wednesday, saying "rogue elements" in the intelligence services were briefing against the government.