Ministers say the terms of the inquiry are far-reaching
The Conservatives are to step up the pressure on the government to hold the Iraq war inquiry in public by staging a Commons debate on the issue.
MPs will debate a Tory motion next week calling for the Iraq proceedings to be held in public "whenever possible".
The opposition also wants the terms of the inquiry, to be headed by civil servant Sir John Chilcot, to be revised and its membership widened.
Labour says it must be held in private for national security reasons.
Announcing the long-awaited probe on Monday, Gordon Brown said it would be modelled on the Franks inquiry into the Falklands War in the 1980s, which was held in private.
He said it would cover all government decisions taken on Iraq from July 2001 onwards, including the planning and conduct of the war and the post-war military and civil operations, but it would not seek to "apportion blame".
Ministers say it should be held behind closed doors to protect security and intelligence, to ensure key officials can appear and talk freely and vital documents be considered.
They stress the report will be published in full, containing all but the most sensitive information.
But opposition parties say the gravity of the Iraq conflict, in which 179 British military personnel died, mean the public must be involved to ensure lessons are properly learned.
'Credibility at stake'
The Tories will use their opposition day debate to call for public hearings except when the matters discussed would directly affect national security.
"To have real credibility the inquiry needs to be open to the public whenever possible," said shadow foreign secretary William Hague.
"I hope the government will come to its senses and listen to the sincere objections being made, seek a genuine consensus and revise its proposals for the inquiry."
The motion is likely to be backed by the Lib Dems, who support a public inquiry, and could attract support from some Labour MPs who oppose the government's position.
The motion will also call for the membership of the inquiry committee, which critics say is too narrowly drawn and full of establishment figures, to be "wider and diverse".
In addition to Sir John, the inquiry committee will be made up of former diplomat Sir Roderick Lyne, historians Sir Martin Gilbert and Sir Lawrence Freedman and crossbench peer Baroness Prashar.
Opposition parties are also unhappy about the timing of the inquiry, which Labour says will take a year to complete.
As it stands, the report is unlikely to be published before the next general election.
The BBC's Political Correspondent Jo Coburn said the debate could be difficult for Gordon Brown who suffered an embarrassing defeat in another opposition day debate, on Gurkha settlement rights, in April.