The government has been told to release the minutes of two cabinet meetings in the days before the Iraq war.
The order relates meetings of Tony Blair's cabinet in 2003
The demand came from Information Commissioner Richard Thomas after a Freedom of Information request was rejected by the Cabinet Office.
He said disclosure would "allow the public to more fully understand this particular decision of the cabinet".
The Cabinet Office has 35 days to appeal against the decision and is said to be "considering" its response.
In his ruling, Mr Thomas says the minutes had to be released to help "transparency and public understanding of the relevant issues".
He also says that accountability for the decisions made is "paramount".
The person making the request said that not releasing the information created "a public impression that something not entirely truthful has been uttered".
But the Cabinet Office refused to release minutes on the grounds that the papers were exempt from disclosure as they related to the formulation of government policy and ministerial communications.
However, Mr Thomas ruled that, in this particular case, the public interest in disclosing the minutes outweighed the public interest in withholding the information.
He said he did not believe that disclosure would "necessarily" set a precedent in respect of other cabinet minutes.
Mr Thomas accepted that a number of specific references in the minutes could damage Britain's international relations if they became public and could be "redacted" - blacked out - before the minutes are released.
The ruling is set to reopen controversy over the then attorney general Lord Goldsmith's legal advice on the war.
On the eve of war, 17 March, his 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 into the lead up to war, 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.
'Wall of secrecy'
The government has not yet decided whether or not to appeal the decision of the Information Commissioner that orders them to release the minutes of cabinet meetings that discussed the legal advice on going to war in Iraq.
In response to the Information Commission's decision a government spokesman said they were "considering" it.
He said: "The requirements of openness and transparency must be balanced against the proper and effective functioning of government.
"At the very heart of that system is the constitutional convention of collective cabinet responsibility."
Liberal Democrat foreign affairs spokesman Edward Davey said: "Labour's wall of secrecy over the Iraq war is gradually being dismantled brick by brick.
"The case for an independent inquiry into the decision to go to war is only strengthened by these continuing efforts to delay and obstruct those seeking the truth."