Interest in his appearance is intense: a public ballot was held Monday for public seats at the hearings, and the lucky few will be allowed into either the morning or afternoon sessions.
Last month Blair admitted in a television interview that he would have backed the war even if he knew Iraq did not have weapons of mass destruction, triggering fresh criticism.
Blair, who quit as premier in 2007 and is now the Middle East Quartet's envoy, told the BBC it would "still have been right to remove" Iraqi leader Saddam Hussein because of the threat he posed to the region.
His support for US president George W. Bush over the 2003 invasion caused a major backlash in Britain, contributing to his decision to step down in June 2007 despite having led his Labour party to three election wins.
In evidence Monday, Blair's chief of staff at the time, Jonathan Powell, robustly defended the former prime minister's conduct.
He acknowledged public opposition to the Iraq war, saying the million-strong protest through London just weeks before the invasion "made a big impression on us", and said they had feared it could cost Blair his job.
But Powell said: "He decided to do something that was unpopular because he thought it was right."
However, while he insisted there had been some success in Iraq, he admitted the deaths in the conflict were something that was "very hard to live with".
Previous inquiry witnesses have suggested Blair was committed to removing Saddam Hussein from the beginning, but while Powell accepted the premier had wanted him gone, he said he was determined to act through the United Nations.
The Iraqi leader had repeatedly violated UN resolutions concerning his weapons of mass destruction (WMD) and this was used to justify the invasion, even though there was no explicit approval from the UN Security Council.
At a meeting in 2002 at Bush's ranch in Crawford, Texas, Powell said Blair had promised support for the United States in pressuring Iraq, but said this was in order to gain influence with Bush -- and denied war was inevitable.
"It was absolutely clear we were not signing up to war on this, we were going down the UN route and giving Saddam a chance to comply," Powell said.
He said there had actually been a "huge gulf" between Britain and the Americans on the issue of regime change that they had sought to keep hidden from the public.
Blair had urged the US administration to think through the "unintended consequences" of any military action to depose Saddam, Powell said.
"We were trying to say to them, don't rush into anything... above all build a coalition," he said -- adding that then US secretary of state Colin Powell had shared their view and actually tried to influence Bush through Britain.
The WMD were never found, but Blair's aide disputed suggestions that Britain should have been more sceptical about intelligence related to their existence.
The Iraqi leader had used chemical weapons on his own people, he noted, adding: "We had the assumption that they had weapons of mass destruction, the intelligence confirmed that and we didn't really have any doubts about it."
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