LONDON -- Testifying at a judicial inquiry, British Prime Minister Tony Blair on Thursday denied exaggerating Iraq's arsenal and defended his government's treatment of a scientist who raised questions about the need for war and later committed suicide.

The case has provoked the biggest crisis in Blair's six years in office, testing his credibility and threatening to bring down a senior member of his Cabinet.

Support for the war in Iraq has been further eroded by the military's failure to find weapons of mass destruction there and the continuing violence that has claimed the lives of about 50 British soldiers since March 20.

Appearing at the Royal Courts of Justice, Blair became only the second British prime minister forced to undergo questioning in a judicial inquiry.

Blair told the court that he was indignant about a BBC radio broadcast in May that accused his advisors of inserting dubious claims into a prewar dossier about Iraq's alleged weapons of mass destruction. If the accusations were true, Blair said, "it would have merited my resignation. It was not a small allegation, it was absolutely fundamental."

Blair arrived at the court and cast a steady glance at about 100 jeering protesters wearing Pinocchio noses. He then went inside to testify about the death of David Kelly, a Defense Ministry scientist and former U.N. weapons inspector in Iraq.

In his smooth and direct style, Blair said the BBC broadcast, based on an interview with Kelly, had maligned him and his government.

Blair asserted that a dossier released Sept. 24 making the case that Iraq possessed weapons of mass destruction was prepared by spy agencies based on solid intelligence, including a claim that Saddam Hussein's regime could deploy chemical and biological weapons within 45 minutes.

That statement, based on a report from a senior Iraqi military officer who was a spy for the MI6 intelligence agency, was the target of the BBC broadcast.

Blair told Lord Justice Brian Hutton, who is leading the inquiry, that he took full responsibility for the decision to publicly identify Kelly as the sole source of the BBC report. Critics say Kelly was swept up in a government vendetta against the network, which has been generally critical of the war.

Kelly killed himself last month after becoming distraught over sustained scrutiny by legislative committees, journalists and his bosses at the Defense Ministry. The inquiry so far has indicated that he had reservations about some of the allegations in the dossier, although he denied making some of the comments attributed to him in the broadcast.

Kelly's name had to be disclosed because it was likely to have been leaked, Blair said Thursday. The prime minister testified that he ordered senior officials to handle the matter "by the book" because he feared the government would later be accused of trying to cover up Kelly's admission of his contact with the BBC. The top civil servant in the Defense Ministry recommended concealing Kelly's identity, but he was overruled, according to evidence presented in the hearings.

"I cannot emphasize enough the fact that we thought literally at any moment this information was going to come out," Blair said. "Secondly, because of the sensitivity of this, it was better just to be open about it."

Opposition leaders chastised Blair on Thursday, saying he had adopted an official strategy that treated Kelly as an expendable pawn. The prime minister's account confirmed suspicions that the government is obsessed with image and reckless with facts, they said.

Conservative Party leaderIain Duncan Smith accused Blair of permitting "the systematic attempt to destroy or reduce [Kelly's] reputation in the public domain. And I think all of that, all of the aspects of the treatment of Dr. Kelly, lie at the heart of Downing Street. So that's the most shameful aspect of this whole saga."

Although Blair has come under fire from both the right and the left, he does not have any significant political challengers at the moment. His problems stem mainly from new postwar wariness about his highly personal political approach, which emphasizes morality and principles.

"A lot of people buy into the view that the government did exaggerate the Saddam threat in general terms," said Michael Cox, professor of international relations at the London School of Economics. "If Blair could show weapons of mass destruction, happy Iraqis, oil flowing out of Iraq, it would be different. But the fact that they haven't found WMD, even more than what goes on in this inquiry, is what affects and shapes people's attitudes. It all makes Blair a weaker prime minister than he was a year ago."

Doubts about the reasons for the war have weakened Blair's ability to push difficult initiatives such as health-care reform or adoption of the euro. There is persistent speculation that the repercussions will bring down a senior official, perhaps Defense Minister Geoff Hoon, whose testimony Wednesday has been portrayed in some news reports as evasive.

Nonetheless, Blair's testimony on Thursday appeared to further weaken a central allegation made by the BBC reporter in the broadcast and in a subsequent newspaper column: that the prime minister's media advisor, Alastair Campbell, inserted dubious information about the 45-minute claim into the September dossier despite the objections of spy agencies.