Tony Blair was yesterday confirmed as one of the witnesses who will appear before Britain's long awaited inquiry into the Iraq war as it was launched with a promise to level criticism where necessary.

The former prime minister is likely to be joined by Gordon Brown among those called to give evidence.

Inquiry chairman Sir John Chilcot said his committee would look at the period from the summer of 2001 to the end of July 2009, covering the run-up to the conflict, the military action and the aftermath. This is the widest scope ever for a government inquiry.

"We are determined to be thorough, rigorous, fair and frank to enable us to form impartial and evidence-based judgements on all aspects of the issues, including the arguments about the legality of the conflict," he told a press conference to announce the launch of the inquiry.

Sir John stressed that this was not a court of law and no one was on trial. "But I want to make one thing absolutely clear. 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."

The aim is to better equip any future government to respond in the most effective manner if a similar situation is faced in the future, he said.

One of the first priorities will be to hear from the families of those who died in the conflict and others who were seriously affected. Sir John said he had already written to many of the families explaining what the Committee is doing and asking them what they think the priorities of the inquiry should be.

It will be up to the individual families whether they want to talk and whether they would like the discussion to be in public or private.

Ultimately as much as possible of the inquiry will be conducted in public, including the possibility of hearings being televised and live streaming on the Internet.

Sir John, however, said some evidence sessions will need to be in private to give the inquiry the best chance to get to the heart of what happened and to protect national security.

The first evidence is not expected to be heard until late autumn. Initial witnesses are expected to be less senior individuals and the hope is that they will be held in public.

The committee expects to travel to Iraq at some stage to gather evidence.

Sir John said there is no chance, given the volume of evidence, that the inquiry will draw its conclusions within a year. He said it could take 18 months and maybe even longer, though he does not want the inquiry to drag on.

The timetable makes it highly unlikely that an interim report will be published before an election. The most that could be published is thought to be a “discussion and issues document”.

Sir John, formerly a senior civil servant, said that the five-member committee will initially meet in private to examine documents relating to the preparation and prosecution of the war in March 2003 and its aftermath. The panel also includes Baroness Prashar, a crossbench peer; Sir Lawrence Freedman, Professor of War Studies at King’s College London; Sir Martin Gilbert, a military historian and biographer of Winston Churchill, and Sir Roderick Lyne, the former British Ambassador to Moscow.

The Government has agreed that the inquiry should have access to papers written by the US or other foreign governments which are held by the British Government. The inquiry will also be able to call witnesses who are not of British origin, such as Hans Blix, the UN Weapons Inspector, or Kofi Annan, the former UN Secretary-General.

The secretary of the inquiry will be Margaret Aldred, currently Director General and Deputy Head of the Foreign and Defence Policy Secretariat in the Cabinet Office. She spent 25 years in the Ministry of Defence. A team of ten staff has also been recruited.

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