Defence secretary Bob Ainsworth said he believed all those involved in the UK's decision to go to war would be willing to give evidence voluntarily.
But ex-minister Clare Short, who resigned from the Cabinet over the post-war governance of Iraq, said the inquiry would be an "academic exercise" in which people could decline to answer questions they weren't happy with.
"It won't get to the bottom of things and people won't get the answers they want," she told Channel 4 News.
The reasons for going to war in Iraq - including the now discredited claim that Saddam Hussein had weapons of mass destruction which could be used within 45 minutes of an order being given - have been a source of long-standing controversy.
Two inquiries - the Hutton and Butler inquiries - have already been held into aspects of the Iraq war.
Nick Clegg said a public inquiry into Iraq was needed
The Butler inquiry looked at intelligence failures before the war while the Hutton inquiry examined the circumstances leading to the death of former government adviser David Kelly.
In 2008 the government defeated Conservative attempts to force a public inquiry, saying it would be a "diversion" for UK troops serving in Iraq.
In February Justice Secretary Jack Straw vetoed the publication of minutes of cabinet meetings discussing the legality of the war in the run-up to the invasion.
In March David Miliband, the foreign secretary, said the government was committed to holding a "comprehensive" inquiry into the conduct of the war and its aftermath.
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