Justice Secretary Jack Straw has vetoed the publication of minutes of key Cabinet meetings held in the run-up to the Iraq war in 2003.
He said he would use a clause in the Freedom of Information Act to block the release of details of meetings in which the war's legality was discussed.
Releasing the papers would do "serious damage" to Cabinet government, he said, and outweighed public interest needs.
The Information Tribunal ruled last month that they should be published.
They had rejected a government appeal against the Information Commissioner's ruling that the papers be published because decisions taken in the run-up to 2003 invasion of Iraq were "momentous" and controversial.
The government could have appealed against the Information Tribunal's decision in the High Court, but has decided instead to use the ministerial veto for the first time since the Freedom of Information laws came into force.
Mr Straw told MPs he had not taken the decision - which had to be approved by Cabinet - to block the minutes "lightly".
But he said it was "necessary" in the interest of protecting the confidentiality of ministerial discussions which underpinned Cabinet government and collective responsibility.
"There is a balance to be struck between openness and maintaining aspects of our structure of democratic government," he said.
"The damage that disclosure of the minutes in this instance would do far outweighs any corresponding public interest in their disclosure."
The Conservatives said the decision was "right" since the release of the minutes would make ministers more reluctant to discuss controversial subjects in future, impeding good government.
However, shadow justice secretary Dominic Grieve said the way the government had handled the issue betrayed its contempt for the FOI legislation it itself introduced.
He also repeated his call for a full-scale public inquiry into the Iraq war, saying the need for this was now "overwhelming".
For the Lib Dems, justice spokesman David Howarth said the decision was "more to do with preventing embarrassment than protecting the system of government".
He said it was in the public's interest to know that the Cabinet, as a decision-making body, had "collapsed" in the run-up to war and been supplanted by a handful of key individuals around the then prime minister Tony Blair.
Labour MP Tony Wright said it was a cause of "great regret" that the veto had been used for the first time and would reinforce the impression among the public that there was something that ministers wanted to hide.
The SNP described the move as a "cover-up" and said an inquiry was needed so that lessons could be learnt from the "worst UK foreign policy decision in living memory".
"The public feels it was lied to about the reasons for going to war in Iraq, and those responsible must not be allowed to hide from an inquiry," said its defence spokesman Angus Robertson.
The release of the cabinet minutes would have reopened controversy over 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.
But after long-running reports that he had changed his mind as the planned invasion approached, his initial lengthy advice given to Mr 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.
Mr Blair defended his decision not to show the cabinet the full advice, saying Lord Goldsmith had attended the cabinet in person and was able to answer any legal questions and explain his view.
Information Commissioner Richard Thomas, who backed publication on public interest grounds, said the exercise of a veto over standard FOI procedures must be "exceptional".
"Anything other than exceptional use of the veto would threaten to undermine much of the progress made towards greater openness and transparency in government since the FOI Act came into force," he said, adding that he would issue a report to Parliament about the case shortly.
Freedom of Information campaigners expressed concerns that the use of the veto in this case could set a precedent for less controversial material to be withheld in future.
Maurice Frankel, director of the Campaign for Freedom of Information, said ministers should have either accepted the Tribunal decision's or challenged it in the High Court if they believed they had legal grounds to do so.
In its ruling in January supporting publication, 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 of the general decision-making processes in the cabinet at the time".
The Tribunal concluded that the release of the minutes now would not set a precedent in future cases.
Cabinet minutes are not normally released until at least 30 years after the event.