Conservative leader Iain Duncan Smith called on Blair to explain why he chose to go to war despite the warning by Britain's Joint Intelligence Committee, made a month before the invasion.
"Tony Blair was entitled to make that judgment and to take that decision," said Duncan Smith, who had firmly supported British military action. "But given public concern over the controversy of the September dossier on Iraqi weapons, he should now explain why he did it. The country is entitled to know."
Before the war, Blair repeatedly warned of the dangers of rogue states providing terrorists with illicit weapons.
A parliamentary committee report published Thursday revealed that intelligence chiefs advised on Feb. 10 that there was no evidence Iraq provided chemical or biological materials to al Qaeda or other terrorists or that Saddam's regime planned attacks with the weapons. But the assessment warned that going to war would in fact increase the threat posted by al Qaeda and other terror groups.
Liberal Democrat leader Charles Kennedy said it was "highly questionable" whether the House of Commons would have backed the war if it had known of the warning.
"Before the Iraq war, the Liberal Democrats, and others from all across the political spectrum, warned that unilateral military action against Iraq, without international support, risked stirring up even more terrorism," Kennedy said.
"Intelligence chiefs gave the prime minister exactly the same warning privately, yet Tony Blair chose to overrule them."
The disclosure prompted critical headlines Friday. "Blair rejected terror warning," said The Daily Telegraph. "Blair on rack over Iraq war terror warning," said The Times.
Blair's government defended its decision to join the war against Iraq. Health Secretary John Reid, one of the more combative members of Blair's Cabinet, said the government believed Saddam posed a threat either on his own or as a supporter of terrorism.
"The prime minister exercised his judgment having looked at a range of things ... that justified the decision he took. I think he was right," Reid told the British Broadcasting Corp.
Blair had told the Intelligence and Security Committee that he took the warning into account.
"This is where you've just got to make your judgment and it remains my judgment and I suppose time will tell whether it's true or it's not true," the committee reported quoted Blair as saying.
Coalition forces have come under several attacks that bear the hallmarks of terrorist groups since the end of major combat in May, including suicide bombings. U.S. officials have said foreign terrorists are going to Iraq to attack U.S.-led forces.
Also Friday, a senior appeals judge probing the apparent suicide of a government weapons adviser said he would call the director-general of the British Broadcasting Corp. for questioning next week.
BBC Director-General Greg Dyke was called to testify along with three defense officials in the second round of the inquiry chaired by Lord Hutton. Blair and other top officials have already testified.
Hutton is investigating the circumstances surrounding the death of David Kelly, who apparently killed himself after being identified as a possible source of a BBC report that said the government had overstated the threat posed by Saddam Hussein in a September dossier making the case for war. The dispute has become the biggest crisis of Blair's six years in office.