The inquiry into the Iraq war is being launched, amid pressure for as much as possible of it to be held in public.
The Iraq inquiry will be as open as possible with hearings which may be televised or streamed on the internet, chairman Sir John Chilcot has said.
Chairman Sir John Chilcot will outline some details of the inquiry, although its exact terms of remit are not expected to be spelled out.
Launching the inquiry, he confirmed former PM Tony Blair would be called, along with senior and junior officials.
There have already been four inquiries into aspects of the war but critics say there are still questions to answer.
He said it should be "as open as possible" but some hearings would be held in private for national security reasons or to allow "more candour".
Defence Secretary Bob Ainsworth said hearings would be in public unless there were national security concerns.
He said an interim report, which could come before the election, was unlikely.
The inquiry is expected to take about a year and report back after the next general election. It will not be a judicial inquiry - witnesses will not give evidence under oath and will not have legal representation.
It is important that national security should not be regarded as a synonym for government embarrassment Sir Menzies CampbellFormer Lib Dem leader Send us your comments
Former prime minister Tony Blair is expected to be called to give evidence.
But BBC political correspondent Paul Adams said it may be some months before public hearings are held, as the inquiry panel will have to go through all relevant government documents from departments, embassies and the four previous inquiries.
Prime Minister Gordon Brown had said it would not seek to "apportion blame" but later Foreign Secretary David Miliband said it would be free to "praise or blame whoever it likes".
When the prime minister announced the inquiry into the 2003 invasion in June, he said it would be held in private.
Cabinet decisions
But, amid criticism from other parties, campaigners, and some military figures he later said it was up to Sir John to decide to hold parts in public if he wished.
Sir Menzies Campbell was foreign affairs spokesman for the Liberal Democrats - the only one of the main three parties to have opposed the invasion - at the time of the invasion.
He told BBC Radio 4's Today programme: "It is important that national security should not be regarded as a synonym for government embarrassment."
I hope and believe that we can trust Sir John Chilcot to hold as much of the inquiry in the public domain as is consistent with national security Bob AinsworthDefence Secretary
He had argued that witnesses should give evidence under oath and said it was important for public confidence that the inquiry was not "denied important information".
Despite the earlier inquiries, questions remained about cabinet meetings in the run up to the decision to go to war, he said.
He added: "We have never had a proper inquiry into how it was that government came to reach this decision.
"For example how was it that the cabinet was apparently taken along the path towards military action without any kind of resistance except for example from Robin Cook and Clare Short?"
Broad timescale
But Lord Anderson, who chaired the Commons' foreign affairs committee inquiry into the war when he was a Labour MP, said it would be difficult to see "what new evidence can emerge".
He said any "added value" might come from looking at what happened to reconstruction efforts in the aftermath of the war.
Defence Secretary Bob Ainsworth told the BBC: "I hope and believe that we can trust Sir John Chilcot to hold as much of the inquiry in the public domain as is consistent with national security.
INQUIRY MEMBERS Sir John Chilcot (chair)Sir Roderick LyneSir Martin GilbertSir Lawrence FreedmanBaroness Prashar
"There will be issues that he feels he has to go in private, I hope that people understand that but where he can, he will take evidence in public."
The inquiry will cover the eight years from the summer of 2001 to the present.
Conservative and Liberal Democrat MPs have pushed for changes to the inquiry panel to include people with military and ministerial experience.
The panel is made up of Sir John, a former civil servant, former diplomat Sir Roderick Lyne, historians Sir Martin Gilbert and Sir Lawrence Freedman and Baroness Prashar.
Ministers resisted holding an inquiry until the bulk of British troops had left the country. There are about 150 troops left in Iraq helping with training and other non-combat tasks.
Among those who have warned against a private inquiry are Lord Butler, the author of the last official report into the Iraq war, which examined government use of intelligence, in 2004.
Previously, the Hutton inquiry had looked into the circumstances surrounding the death of government advisor David Kelly.
There were also two inquiries by the foreign affairs committee and the intelligence and security committee.
