The order concerns meetings of Tony Blair's cabinet in 2003
Ministers have been ordered to release minutes of the cabinet meetings which discussed the 2003 invasion of Iraq.
The Information Tribunal upheld a decision that details of the March 13 and 17 sessions should be disclosed.
The sessions covered whether invasion was allowed under international law. Ministers failed to block the Freedom of Information bid to release minutes.
Downing Street said it was considering its response. The Lib Dems and Tories repeated calls for an Iraq war inquiry.
The Cabinet Office now has 28 days to decide whether to appeal to the High Court against the ruling.
Alternatively, the government could decide to veto the request under Section 53 of the Freedom of Information Act within 20 working days of the tribunal's ruling.
Cabinet minutes are not normally released until at least 30 years after the event - but the Tribunal stressed that disclosure of the Iraq material would not necessarily set a precedent.
The Tribunal said: "The decision to commit the nation's armed forces to the invasion of another country is momentous in its own right, and ... its seriousness is increased by the criticisms that have been made (particularly in the Butler Report) of the general decision-making processes in the cabinet at the time.
"There has also been criticism of the attorney general's legal advice and of the particular way in which the March 17 opinion was made available to the cabinet only at the last moment and the March 7 opinion was not disclosed to it at all."
Information Commissioner Richard Thomas said he was "pleased" the Tribunal had upheld the decision he made in February last year that "the public interest in disclosing the official cabinet minutes in this particular case outweighs the public interest in withholding the information".
He added: "Disclosing the minutes will allow the public to more fully understand this particular decision."
Liberal Democrat foreign affairs spokesman Ed Davey welcomed the tribunal's ruling saying it could be "critical" to how the decision to go to war is viewed by history.
He denied releasing documents before the normal 30-year time limit risked damaging the effective running of government, arguing that many people were still "angry" about the Iraq war.
"The people who took these decisions, which were incredibly controversial, should be held to account," he told the BBC News channel.
"And unfortunately the Labour government has put up a wall of secrecy, in the years since 2003, and prevented the full facts from coming out."
He repeated the Lib Dems' call for a full inquiry into the Iraq war, something the government has agreed to in principle without setting a date for it.
For the Conservatives, William Hague said a full-scale inquiry would be one of the first acts of a Tory government.
But he told the BBC: "I don't think in general that cabinet minutes should be released a few years after the decisions that was made. Ministers need to be able to speak to each other frankly and if this happens regularly well cabinet minutes will become much less revealing over time."
He said it would be more useful to have an inquiry with the power to look at secret documents, informal notes taken and to interview the people involved "so that we can learn the lessons of what happened in the origins and conduct of the war of the mistakes that were made".
The release of the cabinet minutes would reopen controversy over the then attorney general Lord Goldsmith's legal advice on the war.
On the eve of war, 17 March, Lord Goldsmith's opinion unequivocally saying military action was legal was presented to cabinet, MPs and the military and published.
However, after long-running reports that he had changed his mind as the planned invasion approached, his initial lengthy advice given to Tony Blair on 7 March was leaked and then published in 2005.
This advice raised a number of questions and concerns about the possible legality of military action against Iraq without a second UN resolution and was never shown to the cabinet.
The then prime minister Tony Blair defended his decision not to show the cabinet the full advice, saying that Lord Goldsmith had attended the cabinet in person and was able to answer any legal questions and explain his view.
But BBC political editor Nick Robinson said even if the minutes were published, some might be disappointed as by tradition they only list points made, without saying who made them.