Gordon Brown on Monday said the long-awaited inquiry into the Iraq war would be conducted in secret, drawing criticism from opposition leaders who insisted the proceedings must be held in the full glare of public scrutiny.

As Britain prepares for the final withdrawal of its forces from Iraq next month, the prime minister told the House of Commons the inquiry would be independent of government and with the widest possible remit to investigate Britain’s role in the war.

Mr Brown said the inquiry, to be chaired by Sir John Chilcot, a former civil servant at the Northern Ireland Office, would examine British policy from the summer of 2001 until July 2009. Mr Brown did nothing to limit Sir John’s terms of reference, other than to say that the inquiry “will not set out to apportion blame or consider issues of civil or criminal liability”.

Sir John will publish his findings when he has completed his work in one year’s time. However, the announcement that the inquiry hearings would be held in secret – along the lines of the Franks investigation into the origins of the 1982 Falklands war – drew an angry response from opposition leaders.

David Cameron, leader of the Conservative party, said some sessions should be held in public. “Isn’t that what many will want and many will expect, and part of the building of public confidence that is absolutely necessary?” he asked Mr Brown.

Nick Clegg, Liberal Democrat leader, took a similar view, saying: “It looks to me suspiciously like you want to protect your reputation and that of your predecessor instead.”

Mr Brown said that the inquiry – which will be completed after the next general election – must be held in closed session to protect national security. “In this way . . . evidence given by serving and former ministers, military officers and officials will, I believe, be as full and candid as possible,” he said.

To underscore the independence of the inquiry team, none of its members will be former politicians or soldiers. They will include Baroness Usha Prashar, chair of the Judicial Appointments Commission; Sir Roderick Lyne, former UK ambassador to Moscow; Sir Lawrence Freedman, professor of war studies at King’s College London; and Sir Martin Gilbert, the historian best known as the biographer of Sir Winston Churchill.

The inquiry will be Britain’s fifth official investigation into the Iraq war since 2004. The Butler inquiry of that year has until now been the most significant. It inquired at length into the use by Tony Blair, former prime minister, of secret intelligence material to justify the claim that Saddam Hussein possessed “weapons of mass destruction”.

The new inquiry will probably shine most light on the inadequate preparation by the UK and US for a long-term occupation. None of the previous inquiries has looked in any detail into this aspect of the war.

Richard North, author of a recent book on the Iraq war, said: “Illuminating the conduct of the occupation is of vital and immediate importance. This is because the lessons learned . . . have immediate application to the prosecution of the counter-insurgency in Afghanistan.”