The probe will seek access to government records and will also hear from the families of the 179 British troops who have died in Iraq since 2003.

"The inquiry is not a court of law and nobody is on trial," Chilcot said.

"But I want to make something absolutely clear - the committee will not shy away from making criticism."

The inquiry's report is not expected until late 2010 at the earliest, which has prompted complaints that the findings will not be published before the next general election, which has to be held by the middle of next year.

Embarrassing U-turn

The inquiry was announced by Gordon Brown, the British prime minister, last month, honouring a pledge that the run-up to and conduct of the US-led and UK-backed conflict would be examined once British troops pulled out.

Blair faced intense public hostility after backing George Bush over the invasion [GALLO/GETTY] 
Brown's predecessor Blair faced intense public hostility after backing George Bush, the then US president, over the invasion and Blair's resulting unpopularity was one of the main reasons which led to him resigning in 2007.
 
The former prime minister has said he will co-operate "fully" with the probe which will cover events from the summer of 2001 to the end of July 2009.

Brown was forced into an embarrassing U-turn after initially announcing that the inquiry would be held in private, provoking angry claims that it would not be transparent.

Chilcot stressed that the inquiry will be heard in public wherever possible, adding that it could be televised and streamed live on the internet.

But some evidence will be taken in private for national security reasons and to ensure "complete candour", he said, adding that although witnesses could not be compelled to give evidence, he did not expect anyone to decline.

Brown also had to backtrack on an initial pledge that the probe would not "apportion blame," with David Miliband, the foreign minister, telling MPs it could "praise or blame whoever it likes".

Previous inquiries

There have already been two main official probes in Britain into elements surrounding the run-up to the invasion.

The 2004 Hutton inquiry looked at the suicide of David Kelly, a government scientist named as the possible source of a BBC report claiming the government "sexed up" a dossier on Iraq's military capability.

Meanwhile, the Butler inquiry, which reported the same year, highlighted failings in intelligence over whether Saddam Hussein, the Iraqi president at the time, had weapons of mass destruction.

Chilcot was a member of the panel which oversaw that inquiry.