Public opinion 'ignored'
There had been increasing pressure from the public, opposition politicians and members of the ruling Labour party for an investigation into the war which was launched in March 2003.
Many politicians and some relatives of UK soldiers killed in Iraq had called for the inquiry to take place in public.
Lindsey German, from the Stop the War Coalition which protested against the war, told Al Jazeera: "There is no reason this shouldn't be a public inquiry.
"It's carried out by the privy council which is part of the establishment and therefore won't be genuinely independent of the government.
"We have to have an inquiry which asks what Tony Blair and George Bush discussed a year before they took us to war when they met at Bush's ranch in Crawford, Texas.
She said people needed to know "why they drew up a dossier that turned out to be false, as there were no weapons of mass destruction [and] why Tony Blair thought it permissable to ignore British public opinion which demonstrated on a mass scale against this war."
Rose Gentle, whose 19-year-old son Gordon was killed serving in Iraq in 2004, said: "My family and most of the families who lost loved ones just want a simple answer to a simple question - why did we go in to Iraq in the first place?''
|Blair, left, was widely lampooned as a poodle of Bush, right, for his support over the war [EPA]
David Cameron, the leader of the main opposition Conservative Party, said some of the evidence should be held in public and that "if mistakes were made, we need to know who made them and why they were made".
The investigation will be chaired by Sir John Chilcot, 70, a former permanent under-secretary of state at the Northern Ireland Office.
Chilcot has been chairman of Britain's Police Federation since 2001 and sat on the Butler inquiry, which reported into the intelligence which the UK government had about Iraq and weapons of mass destruction.
Brown, who cited national security issue for not holding the investigation in public, said: "The primary objective of the committee will be to identify lessons learned.
"The committee will not set out to apportion blame or consider issues of civil or criminal liability.
"It [the inquiry] will consider the period from summer 2001 before military operations began in March 2003 and our subsequent involvement in Iraq right until the end of July this year."
The war was deeply unpopular in Britain, prompting some of the country's largest-ever protest marches, including a rally which drew an estimated two million demonstrators onto the streets of central London.
Tony Blair, who was prime Minister at the time, was badly tarnished by his decision to join the war and widely lampooned as being a "poodle" of George Bush, the then-US president.
A total of 179 British military personnel have died since March 2003 while serving in Iraq.