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."
In a now infamous claim, Mr Blair told MPs 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.
Sir John'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.
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, 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".
Early in his premiership Mr Blair famously said he was a "pretty straight sort of guy".
In a 1997 interview he said he had been "hurt and upset" about suggestions of dishonesty around the Formula One donations scandal.
"I think most people who have dealt with me think I am a pretty straight sort of guy, and I am," he said.
On Wednesday it emerged a former chief-of-staff of the Iraqi army is seeking to bring a private prosecution against Mr Blair over the war.
General Abdul Wahed Shannan Al Rabbat alleges Mr Blair committed "the crime of aggression" by invading Iraq.
Alongside the former PM, the general has mounted a legal fight against two other key ministers at the time, then foreign secretary Jack Straw and then attorney general, Lord Goldsmith.
Westminster Magistrates' Court refused to issue summonses in November citing immunity granted to former ministers.
The general has applied to the High Court in London for permission to seek judicial review of the magistrates court decision.