LONDON - The head of an inquiry into Britain's role in the Iraq war vowed Thursday not to shy away from criticising government decisions as he opened a probe which will quiz key figures like Tony Blair.

Ex civil servant Sir John Chilcot insisted the probe would not be a whitewash, adding it would visit Iraq and hold discussions with its officials plus those from the United States and other countries involved in the conflict.

"The inquiry is not a court of law and nobody is on trial. But I want to make something absolutely clear -- the committee will not shy away from making criticism," he told reporters.

"If we find that mistakes were made, that there were issues which could have been dealt with better, we will say so frankly."

But opposition politicians and anti-war campaigners fear that those giving sensitive evidence could do so in private.

The Stop The War Coalition, which has led a series of huge anti-war marches in London in recent years, said Blair, premier at the time of the US-led invasion, would "almost certainly" give his behind closed doors.

"The British public has the right to know how and when the decision to go to war was taken, but crucial witnesses will give evidence in private and none will be under oath," it said in a statement.

Chilcot said hearings would be public wherever possible and could even be televised and streamed live on the Internet.

But he added some evidence would be taken in private for national security reasons and to ensure "complete candour".

Although witnesses cannot be compelled to give evidence and will not have to swear an oath, Chilcot said he did not expect anyone to decline an invitation to take part.

He added he would call as witnesses those "best placed to supply the information we need... that will, of course, include the former prime minister".

Blair, who has vowed to cooperate "fully" with the probe, faced intense public hostility after backing then US president George W. Bush's in the 2003 invasion.

His resulting unpopularity was one of the main factors which led to him quitting in 2007.

His successor Brown announced the probe last month, honouring a pledge to hold one after British troops had pulled out.

Britain is currently withdrawing all but a handful of its forces from Iraq, where it had around 45,000 there at the height of the conflict in 2003.

Brown initially said the inquiry would be held in private but was forced into an embarrassing U-turn within days. He also had to backtrack after saying it would not "apportion blame".

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

There have already been two main official probes in Britain into elements surrounding the run-up to the invasion to topple Saddam Hussein's regime.

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 had weapons of mass destruction. Chilcot was a member of the panel which oversaw that inquiry.

Critics have labelled previous probes a whitewash and want answers to questions including why ministers thought incorrectly that Iraq had weapons of mass destruction and what advice it received on the legality of the conflict.

But Chilcot insisted that the report should be judged on its findings, highlighting the credentials of he and his fellow panel members.

"I don't think any of us, candidly, are prisoners of a Whitehall culture... subservient to the government of the day," he told reporters.

The report is expected by late 2010 at the earliest.

It will not be published before the next general election, which has to be held by the middle of next year, and which polls say Brown's government faces an uphill battle to win.