Prime Minister Gordon Brown will give evidence in public later about his role in the events that led to the Iraq war, BBC reported.
As then-chancellor, inquiry witnesses have said he played a key role. It has also been claimed he cut defence funding after the 2003 invasion.
Mr Brown said last month that weapons of mass destruction were not the main reason he backed the war - it was Iraq's disregard for UN resolutions.
Mr Brown will face two sessions lasting two hours in the morning and afternoon.
The inquiry is examining events from 2001 including the decision to go to war, whether troops were properly prepared, the conflict and what planning there was for its aftermath.
A ballot was held to allocate seats at Mr Brown's appearance, as happened with his predecessor Tony Blair's evidence session in January.
Originally the prime minister had been due to give evidence in public after the general election - widely expected to be held on 6 May - but it was moved forward amid some political pressure.
Mr Brown has said he is happy to give evidence early as he did not want people to think there were any "unanswered questions".
The Times reported on Thursday that some families of soldiers who were killed in Iraq had urged the inquiry to challenge Mr Brown over funding for armed forces equipment - particularly the use of Snatch Land Rovers.
In previous evidence sessions former defence secretary Geoff Hoon said Mr Brown had been a key figure in the decisions that led to war.
And the PM is likely to be asked about claims made to the inquiry by Sir Kevin Tebbit, former top civil servant at the Ministry of Defence, that, as chancellor, Mr Brown "guillotined" military spending six months after the invasion.
Mr Brown has said defence spending increased to meet the challenge of the Iraq campaign.
Only one family who lost a relative in Iraq has applied for seats at Mr Brown's hearing - 40 seats were reserved for families over the course of Mr Blair's day-long session.
Last month Mr Brown told Tribune magazine the threat of weapons of mass destruction had not been the main reason he backed the war - it was Iraq's disregard for UN resolutions which had "put at risk" global security.
He added that he wanted "my opportunity to explain what I tried to do".
But the Liberal Democrats, who as a party opposed the Iraq war, dismissed Mr Brown's comments as a "shaky attempt to rewrite history".
Speaking on BBC One's Question Time on Thursday, Lib Dem peer Baroness Williams said: "He was the second most important person in that cabinet - the chancellor of the exchequer.
"He had to sign the cheques to make the war possible. We still don't know whether, deeply in his heart, he felt the war was wrong but he had to stick with Tony Blair as the prime minister, or whether he felt the war was right - I believe that is the central question he has to be asked."
Transport Secretary Lord Adonis told the programme: "All members of the cabinet who were ministers at the time of the invasion of Iraq bear equal responsibility.
"Gordon Brown will not be ducking that responsibility, he will be giving a full account of why he did support the decision that was taken when he appears before the Iraq inquiry."
Lord Adonis said the fact an "exhaustive" inquiry was being carried out showed "that we do not believe as a government we have anything to hide".
Former prime minister Tony Blair gave evidence to the inquiry in January.
He said he had "no regrets" about removing Saddam Hussein from power and insisted the Iraq war had made the world a safer place.