Announcing its decision on Tuesday, 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."

A spokesman for Gordon Brown, the British prime minister, said: "We are considering our response".

Blair criticised

Tony Blair, prime minister at the time of the invasion, was widely criticised for backing George Bush, the then US president, in invading Iraq to oust Saddam Hussein despite failing to secure a second United Nations resolution on the matter.

Ministerial discussions focused notably on Peter Goldsmith's, the then attorney general, advice on the legality of war.

Blair's government strongly resisted demands for the advice of 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.

The information tribunal said that "there has... 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."

The tribunal ruling backed up an earlier decision by Richard Thomas, the information commissioner.

Thomas said: "I am pleased that the tribunal has upheld my decision that the public interest in disclosing the official cabinet minutes in this particular case outweighs the public interest in withholding the information.

"Disclosing the minutes will allow the public to more fully understand this particular decision."