Blair spoke after he and senior officials received an advance copy of the findings from a six-month inquiry into the quality of intelligence on Saddam Hussein's arsenal. The report is to be released to opposition politicians and the public on Wednesday.
Asked at a news conference whether he believed he was given "duff intelligence" before the war, Blair said: "I'm afraid I don't accept that at all."
He offered no comment on the report, saying, "You'll have to wait." His spokesman earlier said the government will not make a formal statement before Lord Butler, who headed the investigation, makes the report public.
Blair, at a news conference with visiting Italian Premier Silvio Berlusconi, also repeated his belief that Iraq was better off without Saddam. The transfer of sovereignty last month had brought some improvement, he asserted, but it was "far too early to be anything other than immensely cautious."
Last week, a Senate Intelligence Committee inquiry concluded that most of the CIA's claims on Saddam Hussein's alleged arsenal were overstated or unsupported. The committee chairman, noting that the United States was not alone in its beliefs, called it a "global intelligence failure."
Blair's office has declined to comment on the findings, saying the "Senate report is a matter for the U.S."
Butler and his team interviewed officials from Britain's main spy agencies, reportedly took evidence from former U.N. chief weapons inspector Hans Blix, and scoured reams of intelligence reports. Their aim was to explain the gap between "the intelligence gathered, evaluated and used by the government before the conflict" and the lack of evidence on the ground in Iraq.
The inquiry also studied an intelligence dossier published by the government in September 2002, as it built a case for war. The dossier, already the subject of three previous inquiries, claimed Saddam had stockpiles of chemical and biological weapons; could deploy them within 45 minutes; and was trying to buy uranium in Africa to develop nuclear weapons — assertions that appear to have collapsed.
The Butler review focused on the "structures, systems and processes" of how that intelligence was gathered and assessed.
According to reports, Butler is expected to censure John Scarlett, chairman of the Joint Intelligence Committee, who signed off on the dossier. ITV news, citing unidentified sources, added that that Butler was not looking for a scapegoat, and would say that intelligence failures were collective.
Blair's office said Tuesday the prime minister had full confidence in Scarlett, who has been chosen to become the next head of Britain's foreign spy agency, MI6.
Blair based his case for war in Iraq on the confident assertion that Saddam had stockpiles of chemical and biological weapons and was a "serious and current threat."
The Iraq Survey Group has, however, found no such weapons. The prime minister last week acknowledged that WMD may never be found in Iraq.
"We do not know what has happened to them. They could have been removed, they could have been hidden, they could have been destroyed," he said.
The "45 minutes" claim was central to a bitter row last year between Downing Street and the BBC over whether the government had, in the broadcaster's words, "sexed up" intelligence.
A government scientist who was the source for the BBC report committed suicide. An inquiry largely blamed the BBC for flawed reporting, leading to changes at the top of the news agency.
The Africa claim was apparently what President Bush was referring to in his 2003 State of the Union speech when he said, "The British government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa."
The White House eventually retracted that now infamous 16-word allegation, after a former U.S. diplomat revealed that he had traveled to Africa to probe one of the reports of Iraqi attempts to buy uranium, and found little to substantiate it.
The Justice Department is probing whether a White House official broke federal law by naming the diplomats' wife, a CIA officer, to reporters.
The FBI is probing forged documents that may have underlay the uranium allegation.
Blair has weathered a nearly constant political crisis since before the March 2003 invasion over his support for Mr. Bush in Iraq. Large parts of his Labor party opposed the war, and questions have only grown after the failure to find weapons, the growth of the Iraqi insurgency and the Abu Ghraib prison scandal.