The Chilcot inquiry's purpose was for the British government to learn lessons from the invasion and occupation or Iraq, in which 179 British soldiers died.
However opponents of Blair's decision to join the war will be pouring over the report for its judgement on how the Labour leader, who quit in 2007, justified the military action.
At the time, he said intelligence showed that Saddam had weapons of mass destruction but after the invasion none was found.
One of those watching the outcome will be Janice Procter, whose son Michael Tench died in the line of duty in Iraq.
Tench was 18 when he was killed in a roadside bomb attack.
Ms Procter hoped the next thing to emerge will be the truth about a war her family believes was illegal.
"Absolutely outraged to be truthful," she told SBS.
"It’s disgusting. I despise this man. Blair is the Iraq monster. To me he sent our kids to slaughter, This man is pure evil."
Sergeant Bob O'Connor died alongside nine others, including Australian Paul Pardoel, when their inadequately protected Hercules aircraft was shot down.
"I think he’d be absolutely devastated and I think he’d be absolutely disgusted," his sister Sarah O'Connor said.
"We went in in a rush, we went in without even the basic or adequate equipment. It was almost staged so that Tony Blair got what he wanted, he made his mind up, and I want to see that man take accountability and responsibility for the devastation.
"While the world sees a soldier all brave and strong, actually, what happens is that a mother buries her baby. And we have to make sure that we don’t do that without there being due diligence paid."
Blair has always rejected any suggestions he acted dishonestly, and a spokeswoman for him said she would not comment until the report was published.
"We went in in a rush, we went in without even the basic or adequate equipment. It was almost staged so that Tony Blair got what he wanted."
It will be released on Wednesday, seven years after the inquiry was set up when the last British combat troops left Iraq. The report runs to 2.6 million words - about three times the length of the Bible - and will include details of exchanges Blair had with then US President George W. Bush over the 2003 invasion.
Iraq remains in chaos to this day. Islamic State controls large areas of the country and 175 people died on Saturday in Baghdad's worst car bombing since the US-led coalition toppled dictator Saddam Hussein.
The inquiry, headed by former civil servant John Chilcot, looked at the reasons for the invasion, the war itself and the aftermath - and has taken longer to complete than the British military involvement itself.
Public hearings, including two appearances by Blair, ended in 2011 but since then the writing of the report has been dogged by rows over the release of secret government files and the contacts between London and Washington. It will include details of notes from Blair to Bush and quotes from more than 130 records of their conversations.
Critics believe Blair, who sent 45,000 British troops for the invasion, gave Bush an unconditional promise that Britain would join military action and that he then distorted intelligence to back this up and put pressure on government lawyers to give the invasion legal approval.
The report is expected to be released sometime after 7pm AEST.
Blair's apology
In an interview with CNN last October, Blair apologised that the pre-war intelligence had been wrong and for mistakes in planning, but not for getting rid of Saddam.
He also accepted the war had played a role in the rise of Islamic State but it was far from the only factor.
"I've said many times over these past years, I'll wait for the report and then I will make my views known and express myself fully and properly," Blair told Sky News on Sunday.
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British media said MPs led by the Scottish National Party were considering invoking an ancient law, last used in 1806, to impeach Blair in parliament.
"You cannot have a situation where this country blunders into an illegal war with the appalling consequences and at the end of the day there isn't a reckoning," SNP lawmaker Alex Salmond told Sky News.
The Sunday Times newspaper also reported that Labour leader Jeremy Corbyn - whose own position is in jeopardy after Britain voted last month to leave the European Union - was simply hanging on because he wanted to "crucify Blair".
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