Tony Blair has rejected questions about his credibility as he said errors in pre-war intelligence do not mean the Iraq conflict was unjustified.
Blair said he accepts the report's findings
Opening a debate on the Butler report, Mr Blair told MPs it was still clear Saddam Hussein had posed a threat.
Lord Butler said much of the pre-war intelligence on Iraq was unreliable.
But Tory leader Michael Howard accused Mr Blair of failing to explain why the intelligence had not backed up what the country had been told it said.
The six-hour debate, which began at 1430 BST, also saw Liberal Democrat Charles Kennedy tell Mr Blair he should feel ashamed about the war.
The debate follows Lord Butler's criticisms last week of the way the intelligence was presented in the September 2002 government dossier on Iraq's weapons of mass destruction.
He said the dossier, and Mr Blair's statement to Parliament on the day of publication, may have given the impression of the intelligence being "firmer and fuller" than it was.
The report also revealed some intelligence on Iraq's weapons had now been withdrawn because it was unreliable.
Mr Blair was questioned by Mr Howard about Downing Street's assertion that he only discovered the intelligence had been withdrawn as a result of the Butler inquiry.
Michael Howard accused Blair of "serial ignorance"
Foreign Secretary Jack Straw was told about the withdrawal last September, minister Baroness Symons told peers on Tuesday.
In the Commons Mr Straw said the intelligence referred did not refer to the controversial claim that Saddam could deploy WMD in 45 minutes.
He said it was information that "had not been directly included in the dossier or the JIC assessment but which ... gave some comfort and backing to the assessments which had been made".
"The Intelligence and Security Committee had themselves been told orally by
the head of the secret intelligence service in July about the nature of that
intelligence and the fact that it was being withdrawn," he said.
No evidence of 'distortion'
The Butler inquiry found no evidence of "deliberate distortion or culpable negligence" in the treatment of pre-war intelligence.
And Mr Blair insisted there was "no doubt" from the intelligence assessments that Saddam had the intent, programmes and weapons of mass destruction themselves.
The international community had backed that view in United Nations resolution 1441.
Mr Blair said: "Whatever the situation with actual readily deployable weapons, part of the problem with this is that people have now gone to the other extreme and said there was no threat. That was not the case."
In testy exchanges Mr Blair was challenged by Mr Howard over the report that the intelligence had been branded "sporadic and patchy" by the JIC.
Mr Blair said he fully accepted there had been errors but the decision to go to war had not been a mistake.
Caveats about intelligence would be included if such information was ever published again, he said.
But the failure to include such warnings in the government's 2002 dossier on Iraq's illegal weapons did not mean ministers had tried to deceive people.
Mr Blair said a senior MI6 officer was now examining how to implement its recommendations.
Brian Jones said intelligence mistakes were "staggering"
In future, the Joint Intelligence Committee (JIC) assessments would be published separately from the government's case, he said.
He also announced that Foreign Office official William Ehrman would be the interim new JIC chairman.
Mr Ehrman is currently deputy chairman and will replace John Scarlett, who is becoming MI6 director - although ex-Conservative leader William Hague and former Foreign Secretary Robin Cook both said Mr Scarlett should not take up his new post.
Mr Blair also promised that meetings of ministers and officials on foreign policy would be made more formal - another of Lord Butler's concerns.
The Tory leader said he could not see how any MP, knowing as they did now that weapons of mass destruction were not likely to be found, could have backed the government war motion, which had stressed that Iraq had such arms.
But that did not mean he thought the war was unjustified as Iraq had repeatedly flouted United Nations resolutions.
Mr Howard accused the prime minister of "serial ignorance" about vital issues.
He had not known "vital" details about the claim that Iraq could use biological weapons within 45 minutes, intelligence reports being withdrawn last year and claims of abuse by coalition troops.
"The prime minister's credibility is at stake today," said Mr Howard.
'Hard to say sorry?'
He said Mr Blair "hasn't been straight with the British people
today" and asked "why does sorry seem to be the hardest
The Lib Dems opposed the war and Mr Kennedy said the government would perhaps have found it "mission impossible" to win MPs support if the background to the intelligence had been known at the time.
The public wanted Mr Blair to show "genuine contrition", said Mr Kennedy.