Labor and the Greens said the interim report of the Iraq Survey Group showed that Mr Howard had misled parliament and the people over the reasons Australia went to war.
Senator Bob Brown said the Greens would move a censure motion against Mr Howard in the Senate on Tuesday and he hoped for Labor support.
Labor's foreign affairs spokesman Kevin Rudd said the report was the fourth torpedo to Mr Howard's credibility on the Iraq war and a party spokesman said a number of parliamentary options were being considered.
But Mr Howard continued to defend his decision, saying the Iraq Survey Group had found substantial evidence that Iraq planned to make chemical and biological weapons.
He said the 1,200 strong US-led group, that includes Australian experts, needed more time.
After making an interim report to congress, report leader David Kay told journalists in Washington that no actual weapons had been found.
But that did not mean the US had concluded there were no weapons and it would take another six to nine months to give a firm indication of the state of Iraq's weapons program.
'We have found substantial evidence of an intent of senior level Iraqi officials, including Saddam Hussein, to continue production at some future point in time of weapons of mass destruction,' Dr Kay said.
Mr Rudd said the group's failure to find weapons followed Mr Howard's false pre-war claim that Iraq was trying to reconstitute its nuclear capability by trying to buy uranium from Africa.
He said the prime minister had also falsely claimed that attacking Iraq was necessary to reduce the threat of terrorism, and would lessen the risk of WMD coming into the hands of terrorists, when British intelligence had said exactly the opposite in both cases.
I know Sunzu said war is about attacking the enemy's plans, but this is ridiculous. In the 18 march war speech there was nothing about attacking Iraq's plans for WMDs. What the prime minister said then was:
In the final analysis, the absolute conviction of the government is that disarming Iraq is necessary for the long-term security of the world and is therefore manifestly in the national interest of Australia. The events of the last four months, Iraq�s history, and its 12 years of defiance have convinced the government that the only way to deal with this challenge is by force. Sadly, the government is not surprised that it should have come to this. Force has been the only language that Saddam Hussein�s regime has ever understood.
For 12 years, Saddam Hussein has forced his nation to endure stringent economic sanctions and pariah status rather than give up his weapons of mass destruction. The presence of weapons inspectors has hindered and irritated him but has never stopped his weapons programs. Even during the first four years of weapons inspections, when the inspectors perceived they were making real progress, Iraq continued to develop and successfully conceal biological weapons. Luckily, a series of defectors blew the whistle on some of Iraq�s prohibited weapons programs, forcing the Iraqi regime to reveal one of the most sophisticated and expansive offensive biological programs in the world; but we cannot expect always to be so lucky. Inspectors were ordered out of Iraq before they could finish dismantling it. The available intelligence indicates that, since the departure of inspectors in 1998, Saddam has continued to work on his chemical and biological capabilities and has maintained his nuclear aspirations.
Even under the threat of force he has only engaged reluctantly in token, piecemeal destruction of weapons and continues to deny the existence of weapons programs. Even with over 200,000 coalition troops massed at his borders he quibbles about how interviews are to be conducted with his scientists and how many of the reconnaissance aircraft supporting the inspectors can fly at any one time. After 12 years, he does not believe that the international community has the will to act. In that he has made a terrible error of judgement.
When you look at Howard's actual words he says that Iraq possesses weapons of mass destruction. If he had told the parliament and the country that all Iraq possessed were plans or programmes for weapons of mass destruction he would not have persuaded the country to go to war.
Lastly, the most recent spin:
JOURNALIST: Mr Rudd says you�ve misled the public and misled Parliament.
PRIME MINISTER: Well that�s very interesting, I was doing a bit of research on Mr Rudd, Mr Rudd himself declared on numerous occasions that Iraq had weapons of mass destruction, so Mr Rudd is therefore contradicting himself. Mr Rudd and Mr Crean both said that Iraq had weapons of mass destruction, their disagreement with us was not as to whether the weapons existed but how you dealt with the problem, they wanted it left interminably to the United Nations and if their advice had been followed Saddam Hussein would still be running Iraq.
But then neither Rudd nor Crean had access to the intelligence available to Howard. Most especially Rudd and Crean did not have the access to the cautionary advice which Howard expected would never be made public. We know about the 14 February JIC report and other cautionary advice only because of the Hutton inquiry and the ISC report.
Rudd and Crean have the ultimate and compelling defence. No-one told the opposition leader.