A long-awaited inquiry into the Iraq war began on Thursday with its chairman pledging to cross-examine in public Tony Blair, the former prime minister, and pull no punches with his findings.

Sir John Chilcot, a former senior civil servant who is heading the inquiry, ruled out a “whitewash” and insisted he would not “shy away” from making plain criticisms where necessary.

“If we find that mistakes were made, that there were issues which could have been dealt with better, we will say so,” he said.

The inquiry will examine Britain’s involvement in the war from summer 2001 to the end of this month, covering the run-up to the conflict, the legality of declaring war, the invasion and the aftermath. It is the widest remit given to any inquiry into the war.

Sir John said it posed “quite simply a huge job” and expressed his determination to avoid a “long drawn-out inquiry”. He hopes to publish a report within a year but warned this could slip into 2011.

Many hearings would be held in public to ensure the inquiry was as “open as possible”, Sir John said. The timetable of hearings could yet pose the government a political problem as it might clash with the run-up to a general election.

Gordon Brown, prime minister, was forced into an embarrassing reversal on his decision to hold the inquiry in secret, conceding on the recommendation of Sir John that some, if not all, of the sessions would be open. On Thursday, Sir John said that some evidence would still be heard behind closed doors to “get to the heart of what happened”.

“Sometimes that will be consistent with the need to protect national security, sometimes to ensure complete candour and openness from witnesses.” The panel might recall witnesses to repeat private evidence in public, if deemed necessary.

William Hague, shadow foreign secretary, expressed his concern that the “need for candour” would be used by ministers as an “excuse” to give evidence in private”.

Although the inquiry has no power to force people to testify, Sir John did not expect any key decision makers to decline a request. He argued witnesses would be frank and truthful even without taking an oath. “If anyone were foolish or wicked enough to tell an untruth …and then be found out, their reputation would be destroyed. It simply will not happen.”

The families of military personnel who have been killed or injured in the conflict will be among the first to be asked to contribute.

Nick Clegg, the Liberal Democrat leader, said public hearings were vital to ensure the inquiry was not seen as a “whitewash”.

“It is essential that this inquiry has the teeth it needs to get the job done,” he said.

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