Powell told the Washington Post he still believed the war was "the right thing to do." The Bush administration says its reasons for going to war included not just Iraq's alleged weapons of mass destruction, but also its human rights abuses and destabilizing influence on the Middle East.
Bush administration officials have also pointed to some evidence that Iraq maintained some capacity to build weapons, in violation of United Nations restrictions.
However, Powell indicated the existence of actual weapons was crucial to the case for war. No weapons stockpiles have been found to date.
Asked by The Washington Post if he would have backed the war, Powell said: "I don't know, because it was the stockpile that presented the final little piece that made it more of a real and present danger and threat to the region and to the world."
The "absence of a stockpile changes the political calculus; it changes the answer you get," he told the newspaper.
On Feb. 5, 2003, Powell told the U.N. Security Council that Iraq had "biological weapons and the capability to rapidly produce more, many more," and had shipped "chemical weapons from production facilities out to the field." He also outlined Saddam Hussein's alleged "efforts to reconstitute his nuclear program."
Last week, David Kay, the former head of the Iraq Survey Group that has hunted weapons since the summer, told Congress he has concluded that Iraq did not possess weapons of mass destruction.
"It turns out that we were all wrong, probably in my judgment, and that is most disturbing," Kay said.
On Monday, President Bush said he would appoint a commission to investigate U.S. intelligence failures in Iraq and gaps in other areas, such as secretive regimes like Iran and North Korea and stateless groups such as terrorists.
Mr. Bush defended his decision to go to war on intelligence that Kay now says was erroneous.
"I want all the facts. We do know that Saddam Hussein had the intent and capabilities to cause great harm. We know he was a danger … He slaughtered thousands of people," Mr. Bush said.
The timetable of the probe is unclear, and is a sensitive issue since its findings could become an issue in the presidential campaign.
It is also unclear if the probe will examine merely the intelligence, or also look at how it was used by the president and aides. The president's supporters say any problems with the case for war are due to bad intelligence, while Democrats contend that the Bush administration oversold the intelligence it had.
Overseas, Prime Minister Tony Blair said Tuesday that the British government would also hold an inquiry into the intelligence used in deciding to go to war with Iraq.
"I think there are issues" about intelligence that need to be looked at, Blair said. But he insisted Saddam Hussein had had "weapons of mass destruction capability" when Britain and the United States went to war.
"I have no doubt whatever that we did the right thing," Blair said.
Elsewhere, Australian Prime Minister John Howard acknowledged Tuesday that U.S. and British intelligence on weapons of mass destruction, upon which his government relied to join the war against Iraq, may have been wrong.
"In the fullness of time it might be demonstrated that the advice was inaccurate," Howard said. "We can't be absolutely certain (that the intelligence was wrong). Obviously the evidence is not pointing strongly in the other direction."
Howard's latest comments come amid mounting pressure at home for an inquiry into Australia's intelligence on Iraq.
In the United States, Democrats say they're worried the inquiry planned by Mr. Bush won't be truly independent. Some Republicans worry the inquiry — at least the fifth now underway — will distract the CIA from key tasks.
Senate Minority Leader Tom Daschle, D-S.D., House Minority Leader Nancy Pelosi, D-Calif., and other senior Democrats wrote Mr. Bush, saying "a commission appointed and controlled by the White House will not have the independence or credibility necessary to investigate these issues."
Senate Republicans responded with statements noting the eight-month inquiry of the Senate Intelligence Committee already is well underway.
Committee Chairman Pat Roberts, R-Kan., said he expects the report to answer many questions being asked. However, "if the president has decided to seek advice from such a panel, I will support it," Roberts said.
On Monday, Kay briefed Mr. Bush over lunch at the White House, offering the president "his impressions and what he's learned," White House spokesman Scott McClellan said.
Among many reasons for the mistaken intelligence estimates, Kay has blamed a lack of human intelligence inside Iraq and inadequate funds for U.S. intelligence agencies.
The forthcoming White House investigation comes on top of inquiries by the House and Senate intelligence panels, an internal CIA review, a CIA-commissioned report from retired agency officials and an Army review.
Despite scattered calls for his dismissal, CIA Director George Tenet is considered unlikely to go any time soon, lawmakers and analysts say. But pressure on Tenet could intensify as the House and Senate intelligence committees wrap up their separate inquiries, which are expected to echo Kay's criticisms.
Meanwhile, a senior Russian diplomat suggested Tuesday that the Kremlin's opposition to war in Iraq was vindicated by the failure so far to find any weapons of mass destruction there.