Gordon Brown has been accused during a Commons debate of making a "monumental mess" of setting up the Iraq inquiry.
A Conservative attempt to have the inquiry into the Iraq war heard "whenever possible" in public has been defeated by the government.
Shadow foreign secretary William Hague said the PM had "set out trying to keep it behind closed doors" and said there were still "serious deficiencies".
Gordon Brown was criticised for saying it should be private but has since said some sessions may be held in public.
But Foreign Secretary David Miliband accused the Tories of "sham outrage" and said the inquiry deserved support.
But opposition parties and some Labour MPs wanted more guarantees about the extent of public hearings.
He said that while it was not a "trial or impeachment" it would be free to "praise or blame whoever it likes".
They also wanted MPs to decide on the inquiry's terms of reference and called for evidence to be given under oath.
The Conservatives called the debate after Mr Brown told MPs last week the inquiry would be in private.
The motion was defeated by 299 votes to 260.
Amid much criticism, he later said it was up to the inquiry's chairman, Sir John Chilcot, to hold some sessions in public if he chose to.
'Utterly cynical'
But opposition parties and some Labour MPs want further assurances.
MPs will vote at about 1900 BST on a Tory motion demanding the inquiry be held "whenever possible" in public and calling for its terms of reference to be put to MPs for scrutiny
Shadow foreign secretary Mr Hague told MPs Mr Brown had talked of improving transparency but had produced proposals for a "secretive, behind-closed-doors inquiry".
He added the membership was too restricted while the timing of the inquiry - which is due to report back after the next election - was "utterly cynical and politically motivated".
Since then he said the government had "engaged in a series of climbdowns - a U-turn executed in stages" and had relied on Sir John to announce changes, rather admit it was "in the wrong".
"Now an inquiry that is seriously overdue can't get off to a clean start but will spend an unspecified period of time adjusting its remit - a recipe for confusion rather than clarity," he said.
Franks inquiry
The government has pointed out the Conservatives had been asking for a Franks-style inquiry - a reference to the committee that reviewed the Falklands War - which was partly held in private.
But Mr Hague said several important elements of the Franks inquiry were "completely missing" while the one government had retained - holding it in private - was the one most people regarded as "no longer appropriate".
He said it was "unfair" to Sir John to leave him to clarify all the terms and rules and said they should be put to MPs for scrutiny.
It suits the government to have us believe that the inquiry will be mostly in public, but I have my doubts David HeathLiberal Democrats
Senior Tory Michael Ancram warned that the inquiry's remit could end up being so "open and vague" that Sir John was being asked to "do the impossible".
For the government, Mr Miliband said all parties agreed there was a need for an inquiry adding: "It needs to be comprehensive, independent, not a trial or an impeachment but an effort to learn for the future."
He confirmed it would have access to all cabinet papers, papers from foreign governments and appeared to confirm it would have access to the original legal advice to cabinet on the legality of the war.
But he said it was "right and proper to leave the discretion" on whether evidence should be given under oath, to Sir John.
However Clare Short, the former cabinet minister who resigned as a Labour MP over the war, and former Lib Dem leader Sir Menzies Campbell said Sir John did not have the power to do so and it would have to be decided by MPs.
David Miliband: "It can praise or blame whoever it likes"
Mr Miliband said the inquiry would have "complete freedom to write its own report" but there was a balance to be struck between "speed and confidentiality" and "comprehensiveness and transparency".
Denying reports it was Tony Blair who had pushed for a closed inquiry, Mr Miliband said the former PM had said he had "no problem" answering questions in public.
Asked if the tribunal should be able to attribute blame, Mr Miliband replied: "It is not an inquiry that has been set up to establish civil or criminal liability, it is not a judicial inquiry.
"Everything beyond that is within its remit, it can praise or blame whoever it likes, it is free to write its own report at every stage."
'Predominantly public'
Labour backbencher Gordon Prentice said he was still unclear about how much of the inquiry would be in public.
He said a briefing note circulated to Labour MPs had said the inquiry would "sit in private with scope for public events and hearings" - he believed it should have been the other way around.
Mr Miliband said Sir John had already said "as much as possible" should be public.
The responsibility for the establishment of this inquiry should rest with this House and anything else is a dereliction of our duty Sir Menzies Campbell
But for the Liberal Democrats, David Heath demanded to know the criteria for closed sessions.
He added: "It suits the government to have us believe that the inquiry will be mostly in public, but I have my doubts."
The Liberal Democrats look set to vote with the Tories against the government and some Labour MPs indicated they would do so.
Labour MPs Bob Marshall-Andrews and Paul Flynn both said the key question for the inquiry was whether Britain was "deceived" into going to war in Iraq.
Sir Menzies said Parliament could "save the government" from itself by voting for MPs to set the inquiry's term of reference.
"Anything else is a dereliction of our duty," he said.
Tory leader David Cameron says Tony Blair, Gordon Brown and Alastair Campbell [Tony Blair's former press secretary] must give evidence in public.
Earlier the prime minister's spokesman said Mr Brown would have "no difficulty in giving evidence in public", if national security considerations are met.
