Tony Blair has said he "regrets deeply and profoundly the loss of life" during and after the 2003 Iraq war.
The former prime minister said his refusal to express regret for the decisions that led to war at his first appearance before the committee had been misinterpreted.
But his words were met with cries of "too late" from the public gallery.
Mr Blair also repeated his warning about the "looming" threat from Iran during the four hour session.
Asked whether the coalition's actions in Iraq had made the risk from Iran and other countries developing nuclear weapons worse, rather than better, he said: "I don't think so."
Mr Blair, who is now a UN Middle East peace envoy, said there was "a looming and coming challenge" from Iran.
"I am out in that region the whole time. I see the impact and influence of Iran everywhere. It is negative, destabilising and it is supportive of terrorist groups. It is doing everything it can to impede progress in the Middle East peace process, and to facilitate a situation in which that region cannot embark on a process of modernisation it so urgently needs.
"And this is not because we have done something. At some point - and I say this to you with all the passion I possibly can - the West has got to get out of what I think is this wretched policy, or posture, of apology for believing that we are causing what the Iranians are doing, or what these extremists are doing. The fact is we are not.
"The fact is they are doing it because they disagree fundamentally with our way of life and they will carry on doing it unless they are met with the requisite determination and, if necessary, force."
"I wanted to make that clear, that of course, I regret deeply and profoundly the loss of life, whether from our own armed forces, those of other nations, the civilians who helped people in Iraq, or the Iraqis themselves and I just wanted to say that because it is right to say that and it is what I feel."
Committee chairman Sir John Chilcot had to tell the public gallery, which included relatives of some of those killed in Iraq, to be quiet as some members shouted "too late".
Earlier, Mr Blair revealed that he had privately assured US President George Bush "you can count on us" eight months before the Iraq war.
The private note will remain secret - despite Sir John's calls for it to be published.
He also revealed he disregarded Lord Goldsmith's warning that attacking Iraq would be illegal without further UN backing because it was "provisional".
The ex-PM said he had believed his top legal officer would change his position on whether a second UN resolution justifying force was needed when he knew the full details of the negotiations.
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The prime minister's communications chief Andy Coulson has resigned, blaming coverage of the News of the World phone hacking scandal.
Mr Coulson said coverage had "made it difficult for me to give the 110% needed in this role".
He faced pressure after stories about phone hacking while he was editor.
Mr Coulson quit as editor in 2007 saying he took ultimate responsibility for the scandal but denied knowing phone hacking was taking place.
In a statement on Friday, he said it had been "a privilege and an honour to work for David Cameron for three-and-a-half years".
But he added: "Unfortunately, continued coverage of events connected to my old job at the News of the World has made it difficult for me to give the 110% needed in this role.
"I stand by what I've said about those events but when the spokesman needs a spokesman, it's time to move on."
He said he would leave within weeks and was proud of the work he had done.
In a statement Mr Cameron praised him as a "brilliant member of my team".
The prime minister said: "I am very sorry that Andy Coulson has decided to resign as my director of communications, although I understand that the continuing pressures on him and his family mean that he feels compelled to do so.
"Andy has told me that the focus on him was impeding his ability to do his job and was starting to prove a distraction for the government."
He praised his work and said he wished him the best for the future.
Mr Coulson was editor of the News of the World in 2007 when its royal editor, Clive Goodman, was jailed for conspiracy to access phone messages. Private investigator Glenn Mulcaire was jailed for six months on the same charge.
Mr Coulson denied any knowledge of phone hacking but resigned saying, as editor, he took "ultimate responsibility". A Press Complaints Commission investigation found no evidence that he or anyone else at the paper had been aware of Goodman's activities.
Shadow Chancellor Alan Johnson resigns
After only three months in the post, the news that Alan Johnson was standing down came as a complete surprise at Westminster.
In a statement Mr Johnson said he had taken the decision for "personal reasons" connected to his family.
"I have found it difficult," he said "to cope with these personal issues in my private life whilst carrying out an important front bench role".
No details have yet been released about the nature of these personal circumstances. But unconfirmed twitter chatter suggests there may have been an affair on his wife's part. Or something?
It is understood however that he informed Mr Miliband of his decision on Monday morning and although the Labour leader pressed him to stay, Mr Johnson would not be swayed.
In his resignation letter Mr Johnson praised Mr Miliband's leadership of the Labour Party and said he was confident he would lead the party from strength to strength.
A short time later it was announced Mr Johnson would be replaced by Ed Balls.
The Labour leader paid this tribute to Mr Johnson: "As shadow chancellor and a politician who held five cabinet positions, Alan showed real leadership on issues that mattered to families across our country, warning of the dangers posed by the government's gamble on growth and jobs, promoting educational opportunity and delivering neighbourhood policing."
He added, about his replacement: "Ed Balls is an outstanding economist and is hugely qualified to take our economic message to the country."
Inevitably, there will be speculation as to whether, alongside the undisclosed personal reasons, political factors may also have contributed to Mr Johnson's decision to quit.
Although hugely experienced, Mr Johnson rarely looked comfortable in his brief as shadow chancellor.
There was his light-hearted remark on taking up the post that he would need an economics primer; and then there was his uncertainty when pressed on television over the rate of National Insurance.
A supporter of David Miliband during the Labour leadership contest, Mr Johnson has also previously found himself at odds with his leader over a graduate tax and maintaining the 50p top rate of income tax.
Attention will now shift to his successor Ed Balls.
A key ally of Gordon Brown, Mr Balls has huge experience of economic affairs having spent many years at the Treasury as an adviser to Mr Brown.
However his much more robust opposition to cuts will prompt questions as to how far he may tilt Labour's economic strategy leftwards and away from the cautious approach of Mr Johnson.
Elsewhere, Mr Balls' wife, Yvette Cooper becomes the new shadow home secretary and Douglas Alexander takes over the foreign affairs brief.
It is a fundamental shake-up of Ed Miliband's top team - and hardly the sort of upheaval Mr Miliband would have wanted so early on in his leadership.