Clare Short, who resigned in the aftermath of the war, said in broadcast and print interviews that Blair had "duped" his colleagues about alleged weapons of mass destruction.
"I have concluded that the prime minister had decided to go to war in August sometime and he duped us all along," The Sunday Telegraph newspaper quoted the former international development secretary as saying.
"He had decided for reasons that he alone knows to go to war over Iraq and to create this sense of urgency and drive it. The way the intelligence was spun was part of that drive," the paper quoted her as saying.
Short told the BBC she believed Iraqi scientists had been trying to develop chemical weapons, but she attacked the Blair government's claim that some of those weapons could be launched "within 45 minutes" of an order.
"The claim the stuff was weaponised and might be used in 45 minutes was part of the secret commitment to a date, which meant everything had to be hurried along," Short told the BBC.
Short also accused the Blair government of deceit for the way France had been "vilified" in order to derail efforts to get a new United Nations resolution before the war.
Short resigned last month, angrily accusing Blair of breaking his promise to give the United Nations a central role in the post-war reconstruction and administration of Iraq. She had called his handling of the crisis "deeply reckless" before the war began and threatened to quit then, but stayed on in her Cabinet job through the conflict.
The failure of American and British troops to find evidence that Saddam Hussein had banned weapons is creating a serious political problem for Blair, who said the threat posed by the arms justified war.
Blair reiterated Sunday that the search for illegal munitions would eventually prove him right.
"I have always said to people to wait for the conclusion of this process and there are still masses of sites due to be investigated, but rather than give a running commentary on it we will accumulate the evidence and give it to people," he told journalists traveling with him to a Group of Eight nations summit in Evian, France, Britain's news agency Press Association reported.
"I simply say to people who have already made up their minds, wait for it," Blair added.
Blair told Sky News he already had seen some evidence that was gathered through interviews in Iraq.
Former Foreign Secretary Robin Cook, who quit as leader of the House of Commons to protest the war, told British Broadcasting Corp. radio that it looked as if the government had made "a monumental blunder" in going to war.
"The government should admit it was wrong and they need to set up then a thorough, independent inquiry into how they got it wrong so that it never happens again, we never again send British troops into action on the basis of a mistake," he said.
Foreign Secretary Jack Straw, speaking to BBC television, echoed recent suggestions by U.S. Defense Secretary Donald Rumsfeld that Saddam may have destroyed some of his arms before the war started.
He added that he was confident evidence of the illegal weapons would be discovered and said Short's criticism was off base.
"The intelligence certainly wasn't wrong," Straw told the BBC. "The evidence is there, it is published. They had those weapons systems and they had been building them up."
"We didn't take the military decision on the basis of some contingency as to what we might find later on. We took it on the basis of fully declared and disclosed evidence," he said.
He said Saddam "almost certainly" destroyed some evidence of his weapons.
"I believe there was therefore a pretty substantial effort being put in in the run-up to military action to hide a lot of this stuff and to deceive the international community, even after the military action was over," he said.