"Tony Blair has rejected questions about his credibility as he said errors in pre-war intelligence do not mean the Iraq conflict was unjustified.
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The report also revealed some intelligence on Iraq's weapons had now been withdrawn because it was unreliable.
Mr Blair was questioned by Mr Howard about Downing Street's assertion that he only discovered the intelligence had been withdrawn as a result of the Butler inquiry.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head of the secret intelligence service in July about the nature of that intelligence and the fact that it was being withdrawn," he said."