Number 10 must share the blame for the "collective failure" that meant the UK went to war on the basis of "doubtful" intelligence, Charles Kennedy has said.
Mr Kennedy: Opposed war
But the Lib Dem leader said the narrow remit of the Butler inquiry left key questions unanswered.
He said the legality of the war was still an issue and the attorney general's advice should be published.
Claims Iraq's WMD could be used within 45 minutes was probably used because they were "eye-catching," he added.
He told MPs: "We argued that we wanted the political judgements which informed the decision to go to war placed under the microscope."
Limiting the inquiry to intelligence and its use meant the "most important issue" could not be considered.
"What was the key reality of the political judgement that led the country to war?
"(The US) Congress is continuing to try to get to the bottom of these matters surely the British Parliament should be seen to try to do so as well," Mr Kennedy said.
He asked the prime minister to "square" his acceptance of Lord Butler's finding that Iraq did probably not have any chemical or biological weapons ready for deployment with his own claims about what Saddam Hussein had before the war
This conclusion had surely reinforced that the policy of containment and weapons inspections had been working in Iraq, Mr Kennedy claimed.
He took issue with the prime minister's statement that the 45 minute claim had only take on importance after the war.
This was the claim Saddam Hussein could launch chemical or biological weapons within 45 minutes of an order to do so.
"Huge focus was placed on the 45-minute claim," Mr Kennedy said.
"Surely you will remember the newspapers being full of the 45 minute warning?" he asked Mr Blair.
Someone had to bear the responsibility of putting this claim into the Iraq arms dossier in such an "eye-catching" way - and he called on the prime minister to name the person responsible for inserting the claim.
'War and peace'
Nothing would be able to "erase" the loss of both British and Iraqi lives, Mr Kennedy told MPs.
The Lib Dem leader also reiterated demands for the full advice tendered by the Attorney General on the legality of the war to be made public.
In the light of Lord Butler's criticism of the prime minister's informal style, he asked Mr Blair whether he planned any changes to his management of Cabinet government "for such an important issue as war and peace",
Mr Kennedy, who was against the Iraq war, was speaking in response to a statement by Tony Blair to MPs in the House of Commons about the Butler Report.
The report followed an inquiry into the accuracy and use of Britain's pre-war intelligence on Iraq's weapons of mass destruction.