Lord Butler: Inquiry "dictated more by the government's political interest"
Prime Minister Gordon Brown has told the chairman of an independent inquiry into Iraq that he can decide to hold public sessions if he chooses.
Mr Brown had told MPs on Monday the whole inquiry would be held in private for national security reasons.
But the PM's spokesman said on Thursday he did not object to public sessions.
Meanwhile Lord Butler, the author of the last official report into the Iraq war, said the planned inquiry served the government's "political interest".
Mr Brown had written to inquiry chairman Sir John Chilcot to say that the families of those who died in Iraq may wish to give evidence and it would be up to Sir John to decide at their request whether to hold public sessions, his spokesman said.
Mr Brown had also asked Sir John to consider whether witnesses would give evidence under oath.
On Wednesday various other public figures called for the inquiry to be held in public.
Former Conservative prime minister Sir John Major said the findings risked being denounced as a "whitewash" and said the decision to hold it in private was "inexplicable".
General Sir Michael Jackson, who was head of the Army during the 2003 Iraq invasion, told the BBC it "must be open wherever possible".
And in a report published on Thursday, the Commons public administration committee said: "Inquiry proceedings should as a rule be held in public, with only very limited exceptions to consider the most sensitive evidence.
Simply passing the buck to the committee is not good enough
"Decisions to conduct particular proceedings in private should be made by members of the inquiry itself, not by the government."
Lord Butler, a former cabinet secretary, told peers: "I reluctantly conclude that the form of the inquiry proposed by the government has been dictated more by the government's political interest than the national interest and it cannot achieve the purpose of purging mistrust."
He said the way it had been arranged would ensure "that we will hear no more about it until after the general election" and it must both allow lessons to be learned but also enact a "truth and reconciliation process" for those who felt misled.
On Monday the prime minister told MPs the inquiry would hear evidence in private so witnesses could be "as full and as candid as possible".
He said he was following Cabinet Secretary Sir Gus O'Donnell's advice in holding sessions in private " taking into account national security considerations ... for example what might damage or reduce our military capability in the future".
Lord Butler believes that the inquiry must do more than 'learn the lessons' from the war. There must, he will say, be a 'truth and reconciliation' element to it as well
The prime minister's spokesman, pressed on Thursday about the growing calls for it to be held in public, said: "This is not some protracted Saville-type inquiry that goes on for years involving countless lawyers.
"I think it will be up to Sir John to consider how the precise format of the inquiry should be structured to ensure that the objectives are met.
"The issue for us has always been to ensure that the inquiry is structured in a way that gets to the truth and people are able to speak honestly and candidly about how decisions were taken."
Shadow foreign secretary William Hague said: "This is a step forward in the arrangements for the inquiry, but we need to see a proper U-turn by the prime minister not a half hearted measure, in order to ensure that this inquiry is conducted in the way that the public and Parliament of this country deserve."
Liberal Democrat leader Nick Clegg told the BBC it was an "11th hour U-turn" by the prime minister that did not go far enough.
Sir John Major: "It defies logic to hold most of it in private"
"Simply passing the buck to the committee is not good enough as I warned him personally before he took his decision," he said.
"This inquiry has to be held in public if people are going to reconcile themselves to what happened, understand the facts and move on and learn the lessons for the future."
Lord Hutton, who chaired a 2003 inquiry into the death of Iraq weapons expert Dr David Kelly, is also in favour of the bulk of the evidence in the latest inquiry being heard in public, according to The Independent newspaper.
And the head of the Army, Sir Richard Dannatt, has said he saw "a lot of merit" in holding some hearings in public.
Foreign Secretary David Miliband has said the government did consult opposition parties on the membership of the new inquiry.
But Conservative leader David Cameron warned against "an establishment stitch-up", while the Lib Dems have threatened to boycott the inquiry if it is held in private.
Lord Scott, who conducted the Arms-to-Iraq inquiry in 1992-1996, told BBC Radio 4's The World At One that there was bound to be some evidence that had to be kept private.
But he added: "I take the view that an inquiry, the purpose of which is to allay public disquiet about some event in which the government or the state has been involved, is not going to achieve its purpose unless it is broadly speaking held in public."
MPs will debate a Tory motion next week calling for the inquiry's proceedings to be held in public "whenever possible".
The inquiry will cover the period from July 2001 to July 2009. Hearings will start next month and take at least a year.
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