The Daily Telegraph
Sir John Chilcot suggested that whistleblowers would be able to speak with greater "candour and openness" if their evidence was held behind closed doors.
But critics including Opposition parties said that the public had a right to hear all the evidence presented to the inquiry, which Sir John disclosed was not likely to report back until the end of next year at the earliest.
Tony Blair is the only witness whose attendance at the hearings he would confirm, raising the prospect that the former prime minister could seek to give some of his evidence in secret.
The panel does not have the power to compel witnesses to appear before it, and those who do will not take a legal oath before giving evidence.
But Sir John said that his eventual report would not be a "whitewash" and denied that the five-member panel, made up of four knights and a peer, was too "establishment".
Gordon Brown had originally announced that the inquiry would be held behind closed doors, but, amid public outcry, was forced to agree that Sir John would have the final say over which sessions would be held in public.
The senior privy councillor had repeatedly insisted that he was committed to "openness".
But at a press conference to launch the inquiry, which covers the years running from summer 2001, shortly before the 9/11 terrorist attacks on New York, to this month, when most British troops left Iraq, he admitted that secret sessions would not simply be restricted to matters of national security.
Sir John said: "If the inquiry is to succeed in getting to the heart of what happened and what lessons need to be learned for the future, we recognise that some evidence sessions will need to be private.
"Sometimes that will be consistent with the need to protect national security, sometimes to ensure complete candour and openness from witness."
William Hague, the shadow foreign secretary, said: "It is worrying that ministers and former ministers will still have significant wriggle room if they want to avoid giving their evidence in public.
"If there are difficult truths to be told they should be told in the light of day, not behind closed doors.
"It would be unacceptable if sessions are held in private simply because ministers or former ministers, including Tony Blair, want to avoid embarrassment."
Nick Clegg, the Liberal Democrat leader, added: "Sir John's assurance that evidence will be held in public 'wherever possible' is welcome. But where evidence is given in private, a genuine national security interest must be proven in each case."
Mr Brown had originally intended that the inquiry would not "apportion blame". But Sir John said: "This committee will not shy away from making criticism. If we find that mistakes were made, that there were issues which could have been dealt with better, we will say so frankly."
Rather than a formal oath, Sir John said that he would ask all witnesses to give an undertaking to be "truthful, fair and accurate'' in their evidence.
He insisted that it was "highly unlikely" that any witnesses would refuse to testify before the committee, adding: "The inquiry will have access to all the information held by the Government and may ask any British citizen to appear before it. In the Prime Minister's words: 'no British document and no British witness will be beyond the scope.'"
But Chris Nineham, of the Stop The War Coalition, said: "The British public has the right - and wants - to know who bears responsibility for the decision to go to war.
"And, above all, why should these questions not be asked and answered in public?"
A spokesman for Tony Blair said: "As we've said from the outset, Mr Blair will of course cooperate fully with whatever format Sir John Chilcot sets out for the inquiry."
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