"This is an enormous day and it goes way beyond him and his own reputation," he said.
But Shane Greer, the executive editor of the UK-based Total Politics magazine, said the inquiry was unlikely to really challenge Blair.
"I don't think we're going to see anything particularly new from this," he told Al Jazeera.
"Tony Blair is a tremendous communicator and this is very much a natural environment for him. He's someone who is used to facing very severe questioning."
Blair's decision to go to war in 2003, which saw 45,000 British troops join the US-led invasion of Iraq, prompted huge protest in the UK, dividing the country as well as his own Labour Party.
The inquiry being held in London is likely to focus on the public justification Blair's government gave for the war, notably the so-called dodgy dossier of September 2002.
In a foreword to the dossier Blair wrote that possession of chemical and biological weapons by Saddam Hussein, the then ruler of Iraq, was "beyond doubt" and that he could deploy them within 45 minutes.
But the inquiry has already heard from senior civil servants who said intelligence in the days before the invasion indicated that Saddam's "weapons of mass destruction" had been dismantled.
Blair has always insisted the war was legal, but he has been unable to shake off accusations that intelligence was doctored to support the case for it.
The inquiry was set up last year by Gordon Brown, who took over from Blair as the UK's prime minister, to learn lessons from the war.
It is the third major investigation into the conflict and its members have stressed that it is not a trial.
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Critics of the inquiry say the five-person team, headed by John Chilcot, a former senior civil servant, has been too soft on witnesses.
Peter Goldsmith, Britain's then attorney general who eventually supported Blair's decision to go to war, appeared in front of the inquiry on Wednesday.
He told the inquiry that he originally believed the United Nations had to approve the use of force and only changed his mind a month before the invasion.
Seven years after the invasion, the issue still divides the UK and public interest in the hearing has been such that organisers had to hold a ballot for spectators.
For Blair, the hearing could affect how history will view his career.
Meanwhile, with a general election looming, the Labour Party fears Blair's appearance before the inquiry could reignite public anger and damage their chances in a vote.