Prime Minister Gordon Brown is set to announce details of an inquiry into the Iraq war in the Commons at 1530 BST.
Opposition parties - and many Labour MPs - have been calling for one since shortly after the 2003 invasion.
They will now be watching closely to see whether the long-awaited probe is to be held in public or private.
In March, ministers said they would hold an inquiry into the war "as soon as practically possible" after the bulk of UK service personnel left Iraq.
Mr Brown's spokesman said: "The Prime Minister's statement will be on the end of the UK's military mission in Iraq and an inquiry into the conflict."
A ceremony was held on 30 April in Basra to mark the official end of the six-year British military mission in Iraq.
In 2008 the government defeated Conservative attempts to force a public inquiry, saying it would be a "diversion" for UK troops serving in Iraq.
And in February Justice Secretary Jack Straw vetoed the publication of minutes of cabinet meetings discussing the legality of the war in the run-up to the invasion.
In March David Miliband, the foreign secretary, said the government was committed to holding a "comprehensive" inquiry into the conduct of the war and its aftermath.
Previously, Conservative leader David Cameron has called for a "robust, independent" inquiry into the war similar to that conducted after the Falklands conflict, with a wide remit and the power to question ministers.
Hutton and Butler inquiries
Lib Dem leader Nick Clegg - who has previously threatened that his party would boycott any private inquiry - told BBC Radio 5 Live that the process of any inquiry which was "done openly and enjoys public confidence" was as important as its eventual conclusions.
"The legitimacy of the whole exercise really depends very much on how the inquiry is conducted," Mr Clegg added.
Rose Gentle, whose son Gordon was killed by a roadside bomb while serving in Iraq, told the BBC that she believed the inquiry had to be held in public.
She said: "It can't be held behind closed doors.
"It was our sons that were sent there, our sons that have been killed."
The reasons for going to war in Iraq - including the now discredited claim that Saddam Hussein had weapons of mass destruction which could be used within 45 minutes of an order being given - have been a source of long-standing controversy.
Two inquiries - the Hutton and Butler inquiries - have already been held into aspects of the Iraq war.
The Butler inquiry looked at intelligence failures before the war while the Hutton inquiry examined the circumstances leading to the death of former government adviser David Kelly.