Gordon Brown said the measure has been taken so witnesses can be "as candid as possible".

Mr Brown said the inquiry would be "fully independent of Government", and the final report will disclose "all but the most secret of information".

The move has disappointed opponents of the war who have been pushing for a full public inquiry.

Lindsey German, from Stop The War Coalition said: "My reaction is it is what I expected, another whitewash. It's another inquiry that will take over a year and cost millions of pounds.

"It will bring us no closer to apportioning blame and finding out what happened in this disastrous war in Iraq."

Tory leader David Cameron argued the inquiry should have been started earlier.

He said there was a danger people would think the inquiry had been "fixed" because it would not report until after the next General Election.

Mr Brown said the probe would be conducted by non-politicians, led by ex-Whitehall mandarin Sir John Chilcot.