The prime minister said: "No British documents and no British witness will be beyond the scope of the inquiry."
Mr Brown said the final report would reveal "all but the most secret of information" and the "unprecedented" process would be "fully independent of government".
But the prime minister was jeered by Conservative MPs when he announced that the inquiry would take a year to report - beyond the date of the next general election.
Tory leader David Cameron said that, because of this, there was a danger the public would believe the process had been "fixed to make sure the Government avoids having to face up to any inconvenient conclusions".
Mr Cameron said that the membership of the inquiry "looks quite limited" and complained that its terms of reference were "restrictive".
And Lib Dem leader Nick Clegg - who has always called for the inquiry to be held in public - said that "the government must not be able to close the book on this war as it opened it - in secret".
Mr Clegg added: "To rebuild public trust, this inquiry must be held in public."
Sir John Chilcot, 70, is a former permanent under-secretary of state at the Northern Ireland Office and has been chairman since 2001 of the Police Foundation and sat on the Butler Inquiry into the intelligence on Iraq's weapons of mass destruction.
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.
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.
And 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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