The Prime Minister today announced his inquiry into the Iraq war would be held in private - to the dismay of campaigners.
And its findings will not be published until next summer, after the date of the next General Election - an announcement met by jeers from Opposition MPs.
The Conservative leader David Cameron said there was a danger that by delaying the start of the inquiry and prolonging the term of its deliberations, people would think the inquiry had been "fixed" because it would not report until after the next general election.
Gordon Brown said the probe would be conducted by non-politicians, led by ex-Whitehall mandarin Sir John Chilcot.
It would cover the period from September 2001, in the run-up to war, until July this year when the last UK soldier will come home, Mr Brown told MPs in a Commons statement.
The Premier told MPs that, with the last UK combat mission over in Iraq, "now is the right time to ensure we have a proper process in place to learn the lessons".
He said the investigation would be an "independent privy councillor committee of inquiry" and stressed: "The inquiry will be fully independent of Government. The scope of the inquiry is unprecedented. It covers an eight-year period.
"The committee will have access to the fullest range of information, including secret information. Their investigation can range across all documents, all papers and all material. No British documents and no British witness will be beyond the scope of the inquiry."
Mr Brown said the inquiry was expected to report some time after July next year.
He said that although the inquiry would not take evidence in public, its report would be published with only the "the most sensitive information" held back.
"The primary objective of the committee will be to identify lessons learned. The committee will not set out to apportion blame or consider issues of civil or criminal liability."
Mr Cameron said he was "far from convinced the Prime Minister has got it right". He said: "The whole point of having an inquiry is that it has got to be able to make clear recommendations, go wherever the evidence needs, to establish the full truth and to make sure the right lessons are learned. And it has got to do so in a way which builds public confidence."
The membership of the inquiry "looks quite limited", the terms of reference are "restrictive", the investigation was not specifically charged with making recommendations and "none of it will be held in public".
By not reporting until after the next election, the public will conclude that it was "fixed to make sure the Government avoids having to face up to any inconvenient conclusions".
Mr Cameron set out a series of detailed concerns about the nature and scope of the inquiry.
He said: "This inquiry should have started earlier. How can anyone argue that an inquiry starting six months ago would somehow have undermined British troops?
"Indeed the argument that you can't have an inquiry while troops are still in Iraq has been blown away by the Prime Minister saying today that some troops will be staying there even as the inquiry gets under way."
He called for Mr Brown to look at the possibility of publishing an interim report early next year.
There "has to be a question" about the military experience of the inquiry panel and Mr Cameron called for senior politicians to be involved to consider the political judgments that were made.
He told Mr Brown: "The inquiry needs to be and needs to be seen to be truly independent and not just an establishment stitch-up."
The Conservative leader also questioned why the inquiry would not be allowed to single out people for blame.
"Shouldn't the inquiry have the ability to apportion blame? If mistakes were made, we need to know who made them and why they were made."
He also questioned whether the inquiry would be allowed access to US and Iraqi documents and witnesses.
Mr Cameron called for greater transparency, asking: "Shouldn't there be some proper public sessions?
"Isn't that what many will want and many will expect and part of the building of public confidence that is absolutely necessary?"
Mr Brown last week made a Commons statement about constitutional reform with greater accountability as a key plank of his strategy.
Mr Cameron said: "What happened to that, it hasn't even lasted a week?"
The Liberal Democrat leader Nick Clegg said: "I am staggered that the Prime Minister is seeking to compound the error, fatal to so many of Britain's sons and daughters, by covering up the path that led to it."
Mr Brown said the country could be "supremely proud" of the work done by British troops in Iraq and insisted all the objectives of the mission had been or were being met.
"Thanks to our efforts and those of our allies, over six difficult years, a young democracy had replaced a vicious 30-year dictatorship."
The military mission ended on April 30 and as of today there were fewer than 500 British troops in Iraq, with more returning home each week.
A small number of British Navy personnel, no more than 100, will remain in Iraq for long term training at the Iraqi Government's request.Reuse content