Tony Blair did long-term damage to trust in politics by presenting a case for the Iraq War that went beyond "the facts of the case", Sir John Chilcot has told MPs.
Sir John told the Liaison Committee he "can only imagine" how long it would take to repair the trust.
The author of the 2.6 million word and seven year Iraq Inquiry is being asked about the probe and its findings.
He also said there was no imminent threat to the UK from Iraq in 2003.
The Liaison Committee, made up of the chairmen and women of the various Commons select committees, usually convenes to question the prime minister.
The Iraq War resulted in the deaths of 179 UK military personnel.
Sir John's long-awaited report, published in July, said the UK went to war with ill-prepared troops and "inadequate" plans for the aftermath.
It said policy on the Iraq invasion was made on the basis of flawed intelligence assessments, adding that it was not challenged, and should have been.
Asked if trust in politics had been corroded because MPs were told things that could not reasonably be supported by the evidence, Sir John told the committee: "I think when a government or the leader of a government presents a case with all the powers of advocacy that he or she can command, and in doing so goes beyond what the facts of the case and the basic analysis of that can support, then it does damage politics, yes."
'Powers of persuasion'
On why Mr Blair's cabinet had not challenged his position, Sir John recalled evidence to his inquiry from former foreign secretary Jack Straw.
He said this suggested the former PM, who at the time had won two general elections, "had achieved a personal and political dominance which was in a sense overriding collective cabinet responsibility".
On the case for war, Sir John said Mr Blair did not "state falsehoods knowing them to be false" but had "exercised his considerable powers of persuasion" rather than laying all the evidence before Parliament and the public.
Asked whether the war was legal, he told the MPs the inquiry had not been "in a position" to offer that opinion.
And he said it would be "quite extraordinary" if weapons of mass destruction were found in Iraq, 13 years on from the invasion.
Any discrepancies on the number of WMDs documented were "an accounting problem" rather than a "hidden arsenal", he added.