Sir 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 war.
In an interview with the BBC Sir John was then asked if Mr Blair was as truthful with him and the public as he should have been during the seven-year inquiry.
He replied: "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.
"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.
Sir John's report, published in July last year, 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 Hussein's supposed weapons of mass destruction.
Asked if he felt Mr Blair had given the fullest version of events to the inquiry, Sir John 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."