The unprecedented move came after the Information Tribunal, the country's freedom of information watchdog, last month ordered the government to release the minutes of cabinet meetings from March 13 and 17 2003.
The meetings discussed whether the decision to go to war was allowed under international law.
Tony Blair, the then prime minister, was widely criticised in Britain for backing George Bush, the former US president, in invading Iraq despite failing to secure a second United Nations resolution on the matter.
Ministerial discussions notably covered the advice of Peter Goldsmith, the then attorney general, on the legality of war.
Blair's government strongly resisted demands for the advice of Goldsmith, its most senior legal adviser, to be made public until a large section was leaked during the 2005 general election campaign.
Goldsmith then denied ministers pressured him into changing his mind to rule that invading Iraq would be legal in international law, even without a second UN security council resolution.