Sir John Chilcot : 'The hearings will be held in public whenever possible'
The "huge job" of going through vast amounts of material and evidence means the Iraq inquiry could continue into 2011 says chairman Sir John Chilcot.
Launching the inquiry he said it would be "as open as possible" with hearings televised and streamed online.
But he said some hearings would be held in private for national security reasons or to allow "more candour".
Sir John said Tony Blair would be among those asked to give evidence and said he did not expect anyone to refuse.
There have been complaints that the inquiry, which will cover events from the summer of 2001 to the end of July 2009, will not report back before the next general election.
But Sir John said it was "quite simply a huge job" and the panel had to be given sufficient time to do it thoroughly.
He said he was determined to avoid "a long drawn out inquiry".
But he added: "A period of as little as a year is not going to be enough. So I think late in 2010 is probably going to be the earliest possibility but I don't at all rule out the possibility we may have to go beyond that."
The government must not be able to interfere to keep Blair and Brown out of the spotlight for the sake of political convenience in the run-up to an election
He said he wanted as many people as possible to have access to the hearings - which "includes the possibility of hearings being televised and live streaming on the internet". Transcripts would be available on a special website.
While the inquiry would cover eight years, it would be put in a "historical context".
Sir John said if, on looking through the evidence, they decided an interim report would be appropriate they would publish one. But he said: "It is more likely... that our report will be a single one at the end of the committee's deliberations."
The inquiry's panel has already begun work, making the first requests for government documents.
But the chairman said it could take months to read through the "huge amount" of material and identify issues on which to focus.
Its "first priority" would be to hear from the families of British service personnel killed during the conflict, he said.
Sir John Chilcot, former civil servant
Sir Roderick Lyne, former diplomat
Sir Martin Gilbert, historian
Sir Lawrence Freedman, historian
Baroness Prashar, crossbench peer
All documents held by the British government and any British citizen could be called to give evidence, he said.
The inquiry panel came in with "open minds" and a determination to "review the evidence independently".
He said nobody was "on trial" but the committee would "not shy away from making criticism".
"If we find on going through the evidence that we see ... that people fell short in their duty, made mistakes, acted wrongly, we shall most certainly say so and say so clearly."
He suggested that key figures - likely to include Tony Blair - would appear towards the end of the inquiry to ensure that they were asked the right questions.
Several MPs have said witnesses should be made to give evidence under oath and there have been questions about whether the inquiry would be able to compel witnesses to attend.
Sir John said: "Frankly I don't expect any witnesses whom we invite to refuse to appear - it seems to me highly unlikely."
He said there was no legal basis, in a non-judicial inquiry, to make people give evidence under oath.
I hope this won't become an excuse for ministers and former ministers to hold sessions in private when it is not necessary to do so
William Hague, Conservatives
But he added: "If someone were foolish or wicked enough to tell a serious untruth in front of the inquiry like that, their reputation would be destroyed utterly and forever. It won't happen."
There have already been four inquiries into aspects of the war but critics say there are still questions to answer.
Sir Menzies Campbell said the panel should have included a senior military and political figure and should have appointed a QC to conduct cross examinations.
Shadow foreign secretary William Hague told the BBC he was pleased some sessions would be held in public and the inquiry could "apportion blame".
Mountain to climb
But he said: "I was a little concerned that Sir John has said sessions can be held in private if there is a need for candour.
"We hope the whole inquiry will see a lot of candour and I hope this won't become an excuse for ministers and former ministers to hold sessions in private when it is not necessary to do so."
Liberal Democrat leader Nick Clegg said the inquiry must have "teeth" adding: "The government must not be able to interfere to keep Blair and Brown out of the spotlight for the sake of political convenience in the run-up to an election.
"Tony Blair ordered this disastrous war and Gordon Brown signed the cheques - without public appearances from them this inquiry will be seen as a whitewash."
For the SNP Angus Robertson also said Mr Brown and Mr Blair must give evidence in public.
He added: "Sir John Chilcot has a mountain to climb if he is to convince people that this an open and independent inquiry and not some establishment stitch-up with Downing Street pulling the strings."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.