Secret government discussions about the Iraq war are to be disclosed after an information tribunal today ordered the release of cabinet minutes from 2003.

The decision follows a lengthy battle by campaigners, who have argued that the public interest in learning what was said about the planned invasion outweighs the public interest in cabinet discussions being kept secret.

Ministers have strongly opposed the request, arguing that the Freedom of Information Act was never intended to allow for the publication of information of this kind.

The tribunal upheld a decision by the information commissioner that details of the sessions on 13 and 17 March should be disclosed.

The meetings considered the highly controversial issue of whether the invasion was allowed under international law. Lord Goldsmith, who was attorney general at the time, initially suggested that the legality of the invasion was legally questionable before subsequently issuing legal advice saying that it would be compatible with international law.

This has given rise to persistent claims that ministers were not fully briefed on the possible legal pitfalls of an invasion.

Today's ruling does not necessarily mean the minutes will be published because the government has 28 days to appeal.The tribunal said that the exceptional circumstances relating to the two cabinet meetings meant that publication was justified and that it would not set a precedent for the publication of all cabinet minutes.

Last year Richard Thomas, the information commissioner, ordered the government to publish the minutes. The government appealed to the information tribunal, which adjudicates when official bodies are unwilling to comply with rulings from the commissioner.

Ministers will now have to decide whether to publish the minutes, to appeal to the high court, or to issue a ministerial veto banning publication. Under the Freedom of Information Act the government has the right to refuse publication as a last resort, but this veto power has not been used since the act came into force in 2005.

Thomas said today: "I welcome the careful consideration that the information tribunal has given to this important issue. 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. I am also pleased that the tribunal reached the same conclusion as I did in relation to the publication of the handwritten notes of the meetings."

In its ruling the tribunal said: "We have decided that the public interest in maintaining the confidentiality of the formal minutes of two cabinet meetings at which ministers decided to commit forces to military action in Iraq did not, at the time when the Cabinet Office refused a request for disclosure in April 2007, outweigh the public interest in disclosure. We have reached that decision by a majority and not without difficulty.

"We concluded that there was a strong public interest in maintaining the confidentiality of information relating to the formulation of government policy or ministerial communications (including, in particular, the maintenance of the long-standing convention of cabinet collective responsibility).

"However, this is an exceptional case, the circumstances of which brought together a combination of factors that were so important that, in combination, they created very powerful public interest reasons why disclosure was in the public interest."

Although the tribunal said that the minutes ought to be published, it also accepted that some sections should be redacted, or edited, "to avoid unnecessary risk to the UK's international relations".

The tribunal also ruled that "certain informal notes" taken alongside the formal cabinet minutes did not need to be published.

In its ruling, the tribunal said that the controversy about Goldsmith's legal advice was a factor in persuading it to come down in favour of publication.

"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," the tribunal said.

"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."

The Cabinet Office now has 28 days to decide whether to appeal to the high court against the ruling.

A Downing Street spokesman said: "The information commissioner has just made an announcement on this and we are considering our response."

William Hague, the shadow foreign secretary, said: "Rather than have items of evidence dragged into the public domain piece by piece the government should set up a full-scale privy council inquiry into the origins and conduct of the Iraq war.

"The sooner we can learn the lessons of the war the sooner we can apply them. It is imperative to begin an inquiry before memories have faded, emails have been deleted and documents have disappeared."

Edward Davey, the Liberal Democrats' foreign affairs spokesman, said: "This is a very welcome and long overdue decision. The public clearly has the right to see the minutes. Their publication may help answer some critical questions, and certain people may find themselves in an even more difficult position.

"The government must now accept this decision, and publish the minutes without delay."