Gordon Brown was accused last night of performing a �slow-motion U-turn� after he all but conceded that the bulk of the forthcoming Iraq inquiry should be held in public.

Gordon Brown was accused last night of performing a "slow-motion U-turn" after he all but conceded that the bulk of the forthcoming Iraq inquiry should be held in public.

William Hague, the Shadow Foreign Secretary, made the claim after the publication of an exchange of letters between the Prime Minister and Sir John Chilcot, the ex-Whitehall mandarin leading the investigation.

In a letter to Mr Brown, Sir John said: "It will be essential to hold as much of the proceedings of the inquiry as possible in public, consistent with the need to protect national security and to ensure and enable complete candour in the oral and written evidence from witnesses."

Significantly, the Prime Minister agreed, saying: "Your proposals will manage to meet both the need not to compromise national security but also enable the independent inquiry also to hold public sessions helping to build public confidence."

This is a significant change from Mr Brown's Commons statement last week, when he told MPs that, because of national security, the entire inquiry would have to be held behind closed doors.

This sparked a public outcry led by senior political and military figures, including Sir John Major, the ex-prime minister; General Sir Mike Jackson, former head of the Army; and Lord Butler, the ex-Cabinet Secretary who headed the last official inquiry into aspects of the war. Relatives of the fallen also demanded more transparency, insisting most of the inquiry had to be held in public.

The Herald, in a leader column last Tuesday, said some evidence might have to be heard in secret, if merited by the security and intelligence implications, but, "as a rule, the inquiry should be open".

No 10 sought to defuse the row. It suggested that the UK Government did not have a "theological" attachment to closed hearings and that Sir John would have a degree of discretion in how he conducted proceedings.

Mr Brown then relented a little more by suggesting relatives of the dead could, if they wanted, give evidence in open session.

At the weekend it was claimed that Tony Blair, Mr Brown's predecessor who led the nation into the Iraq war, had urged his successor to keep the hearings secret to avoid them becoming a show trial.

Yet on Sunday, Jack Straw, the Justice Secretary, said he would be prepared to give most of his evidence in public. Yesterday, Ed Balls, the Children's Secretary, argued it would be a "good thing" to hold some of the Iraq war inquiry in open session.

Mr Hague said of Mr Brown's response to Sir John: "The Prime Minister has been executing a U-turn in slow motion ever since announcing the inquiry. Characteristically, he could not bring himself to confirm this in Parliament but has passed the buck to Sir John Chilcot."

He added: "This is a climb-down of massive proportions from the Prime Minister, whose own proposals for the Iraq inquiry were so ill-thought-through and hastily executed that they have attracted nothing but condemnation from all quarters."

Tomorrow, the Shadow Foreign Secretary will lead a Commons debate calling for the membership of the inquiry team to be "wider and more diverse" and for proceedings to be held in public "whenever possible".

Ministerial sources have reportedly suggested Mr Brown is preparing to accept parts of the Conservative motion on the issue of public sessions.