Sir Gus O’Donnell, the head of the Civil Service, will lead the Government’s last-ditch attempt to block the release of minutes of Cabinet meetings in the run-up to the war in Iraq, The Times has learnt.

No 10 has been ordered by Richard Thomas, the Information Commissioner, to release copies of Cabinet minutes and records relating to meetings held between March 7 and 17, 2003.

Details of the meetings, during which the Attorney-General’s legal advice on the war was discussed, could reveal the positions of individual Cabinet ministers and the strength of internal opposition before the March 20 invasion.

The Cabinet Office is fighting the decision because it believes that a vital principle of government is at stake – the right to have private discussions. Ministers argue that releasing the documents could end hundreds of years of confidential Cabinet discussions and undermine collective Cabinet responsibility, where ministers must defend policies in public that they may not agree with in private.

The Cabinet Office says: “If ministers and officials knew or thought that once a decision was reached, information pertaining to the process by which they reached that point was to be revealed, they might be less willing to engage in full and frank discussions of the issues. Their candour in these discussions could be affected by their assessment of whether the content of these discussions will be disclosed.”

Downing Street is taking Mr Thomas’s ruling to the Information Tribunal, the final court of appeal for freedom of information cases, and three days of hearings are due to begin tomorrow. In an unusual move, Sir Gus, who is also the Cabinet Secretary, will make the case personally to underline the concern over the issue inside government.

If the Government loses, one of the Cabinet Office ministers could issue a decree under Section 53 of the Freedom of Information Act unilaterally blocking the release of the document. This would be a highly controversial move – the first time the Government has used this power since the FoI Act came into force in 2005. Furthermore, the judgment could still create a precedent, potentially making it easier for others to request Cabinet minutes.

Two sets of notes are made during each Cabinet meeting. The first is a general printed account of what happened, circulated to Cabinet members afterwards. The second is a handwritten account by the Cabinet Secretary, which contains full details of the individual positions of participants. It is not known whether the handwritten records could be subject to release.

To prevent the disclosure, the Government is relying on two provisions in the freedom of information legislation that should block the release of documents relating to “formulation of government policy” or “ministerial communications”.

However, when the legislation was drawn up by Jack Straw in 2000, he decided that this ban could be overridden if the public interest arguments were strong enough .

The Information Tribunal will have to decide this week whether it agreed with Mr Thomas that “a decision on whether to take military action against another country is so important that accountability for such decision-making is paramount”. It could argue that such a decision would not undermine collective Cabinet responsibility because these were “two specific and unusual sets of Cabinet minutes”.

Only 13 FoI requests to Downing Street have been granted and made public on the Cabinet Office website.

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