John Chilcot has said he does not believe Tony Blair was "straight with the nation" about his decisions in the run-up to the Iraq War.
The chairman of the public inquiry into the 2003 conflict said the former prime minister had been "emotionally truthful" in his account of events leading up to the conflict.
But he suggested Mr Blair, who once described himself as "a pretty straight sort of guy", relied on "belief" rather than "fact".
In an interview with the BBC broadcast on the anniversary of the publication of Mr Chilcot's report, he was asked if Mr Blair was as truthful with him and the public as he should have been during the seven-year inquiry.
"Can I slightly reword that to say I think any prime minister taking a country into war has got to be straight with the nation and carry it, so far as possible, with him or her," he said.
"I don't believe that was the case in the Iraq instance."
A spokesman for Mr Blair told the BBC that "all these issues" had been dealt with.
The Chilcot Report found that former Iraqi dictator Saddam Hussein posed ''no imminent threat'' at the time of the invasion of his country in 2003, and the war was fought on the basis of ''flawed'' intelligence.
While giving evidence to the inquiry, Mr Blair denied he had taken the country to war on the basis of a "lie" over Saddam's supposed weapons of mass destruction.
Asked if he felt Mr Blair had given the fullest version of events to the inquiry, Mr Chilcot said: "I think he gave an - what was - I hesitate to say this, rather, but I think it was, from his perspective and standpoint, emotionally truthful and I think that came out also in his press conference after the launch statement.
"I think he was under - as you said just now - very great emotional pressure during those sessions ... He was suffering. He was deeply engaged. Now in that state of mind and mood you fall back on your instinctive skills and reactions, I think."
In a now infamous claim, Mr Blair told MPs that Saddam Hussein was developing weapons of mass destruction, and later said intelligence showed the Iraqi tyrant could deploy chemical and biological weapons within 45 minutes.
Mr Chilcot's report found Mr Blair presented the case for war with "a certainty which was not justified" based on "flawed" intelligence about the country's supposed weapons of mass destruction (WMD) which was not challenged as it should have been.
Asked if was "exaggerated", Mr Chilcot replied: "He found - I don't know whether consciously or not - a verbal formula in the dossier and his foreword to it.
"He said - and used it again later - 'I believe the assessed intelligence shows beyond doubt.' Pinning it on my belief, not on the fact, what the assessed intelligence said."
The report said the UK chose to join the invasion of Iraq before the peaceful options for disarmament had been exhausted and military action was not a last resort.
It added: "We have also concluded that the judgments about the severity of the threat posed by Iraq's weapons of mass destruction, WMD, were presented with a certainty that was not justified."
Following the report's publication, Mr Blair said that, while the Chilcot Report contained "serious criticisms", it showed "there were no lies, Parliament and the Cabinet were not misled, there was no secret commitment to war, intelligence was not falsified and the decision was made in good faith".