There are still more than 4,000 UK troops in Iraq
The Conservatives have called again for a public inquiry into the "origins and conduct" of the war in Iraq.
Shadow foreign secretary William Hague told MPs that with UK troops likely to have left Iraq by the middle of 2009, a "full-scale" inquiry should follow.
A senior defence source has told the BBC UK forces could start pulling out in March while Gordon Brown has hinted at a near total withdrawal by mid-2009.
Labour has said an inquiry will be needed but only when troops have left.
The Lib Dems also renewed their calls for a public inquiry into British involvement in the overthrow of Saddam Hussein and the role played by UK troops in Iraq since 2003.
The Ministry of Defence said on Wednesday that it anticipated a "fundamental change of mission" in Iraq next year - one expected to result in the exit of the bulk of the 4,100 troops left in Basra.
Defence sources have indicated that should Iraqi elections pass off largely peacefully in January the first phase of troop withdrawal could begin soon after.
Given the imminence of this move, Mr Hague said it was time for the government to "make clear" its intentions over a public inquiry.
"I once again serve notice that if they fail to do so, we will again be returning to the issue this [parliamentary] session," he said during a debate on the Queen's Speech.
He added: "The continued absence of an inquiry, or its setting up on an inadequate basis, will be rectified immediately upon the election of a Conservative government."
Lib Dem foreign affairs spokesman Ed Davey said he hoped the "long-hoped for inquiry" would be announced as soon as possible, saying he would continue to press ministers over its "terms and remit".
"There can be no doubt that an inquiry should have been held several years ago," he told MPs.
Earlier this year, Foreign Secretary David Miliband said there was cross-party agreement that an inquiry would be "necessary".
He said what was in dispute was the issue of "timing not substance". Ministers have said that it would be wrong to hold an inquiry while British troops were still in Iraq.