A newly published document suggests Attorney General Lord Goldsmith had concerns about whether the Iraq war was legal just 10 days before Parliament decided to go to war. On that day his published advice was that the war was legal. The opposition parties suggest that the fact his doubts were not disclosed means that MPs, the cabinet and the public were misled.
The UN Security Council unanimously passed resolution 1441 on 8 November 2002.
It gave Iraq "a final opportunity to comply with its disarmament obligations" and warned Iraq of "serious consequences" if it did not.
It based its authority on resolution 678, which had authorised the war to remove Iraq from Kuwait in 1990 using "all necessary means" to "enforce peace and security," and resolution 687 (passed after the 1991 war) which placed continuing obligations on Iraq to eliminate weapons of mass destruction, and indicated that force against Iraq could resume if they breached these conditions.
As a result, weapons inspectors led by Hans Blix went into Iraq, while 250,000 US and UK troops moved to Iraq's borders.
On 24 February 2003, the UK and the US began an attempt to persuade the Security Council to pass a further resolution authorising the use of force, and US Secretary of State Colin Powell presented evidence that Iraq still had weapons of mass destruction.
On 7 March 2003 Hans Blix told the UN Iraq had taken significant steps to comply with its disarmament obligations, but said there were still unanswered questions. He subsequently said that if asked he would have requested more time to complete his task.
On 17 March 2003 the UK, the US and Spain abandoned their attempt to secure a second UN resolution authorising force, citing the opposition of the French and their threat to veto any such resolution.
We now know that in his advice of 7 March 2003 (on whether the war was legal without a further UN resolution) the attorney general said "the language of resolution 1441 leaves the position unclear."
A "key question" was whether there was a need for an assessment of Iraq's conduct. If so, "it would be up to the Council to make it".
"The safest legal course would be to secure the adoption of a further resolution to authorise the use of force."
However, if the government chose to rely on 1441 alone, a "reasonable case can be made" that it is capable "in principle of reviving the authorisation in 678 without a further resolution" but this "argument is controversial. It is not widely accepted among academic commentators" but "I disagree with those commentators and lawyers".
However, he added that if relying on this argument, "we would need to demonstrate hard evidence of non-compliance and non-cooperation".
And in light of the latest reports by the weapons inspectors, "you will need to consider very carefully whether the evidence of non-cooperation and non-compliance by Iraq is sufficiently compelling to justify the conclusion that Iraq has failed to take its final opportunity".
He also warned that any military action "must have as its objective the enforcement of the terms of the cease-fire" and "must be a proportionate response to that objective, ie securing compliance with Iraq's disarmament obligations".
He added that "regime change cannot be the objective of military action" although removing Saddam Hussein would be legal "if it can be demonstrated that such action is a necessary and proportionate measure to secure the disarmament of Iraq".
However, when the attorney general presented his advice to Parliament and the Cabinet in a note published on 17 March, his legal advice was unequivocal.
"Authority to use force against Iraq exists from the combined effect of resolution 678, 687, and 1441.
"The Security Council also decided that if Iraq failed at any time to comply with and cooperate fully with the implementation of 1441, that would constitute a further material breach.
"It is plain that Iraq has failed so to comply and therefore Iraq was at the time of resolution 1441 and continues to be in material breach.
"Resolution 1441 would have provided that a further decision of the Security Council to sanction force was required if that had been intended. All it requires is reporting to and discussion by the Security Council of Iraq's failures, but not an express further decision to authorise force."
What had changed?
British attempts to get a second UN resolution authorising force had collapsed.
And the military and civil service asked him to "express a clear and simple view" on the legality of military action.
So on 15 March, the attorney general wrote to the prime minister and received written assurance that "it is indeed the prime minister's unequivocal view that Iraq is in further material breach of its obligations".
In a statement released on 28 April 2005, Lord Goldsmith said his 7 March 2003 advice was an outline of the arguments, not a conclusion.
"Far from showing I reached the conclusion that to go to war would be unlawful, it shows how I took account of all the arguments before reaching my conclusion," he said.
The attorney general did not technically change his advice, because he did not come to a firm conclusion on 7 March.
But he expressed no doubts on 17 March that the war in Iraq was legal without a second UN resolution - after Britain failed to get one through.
There is no doubt that if his earlier advice has been disclosed at the time, it would have made it harder for the government to garner support for the imminent invasion of Iraq.
And it is no accident that resolution 1441 itself was ambiguous about whether military action was legal without further resolutions. It had been deliberately crafted that way in order to gain unanimous UN support.
The fact that Britain fought so hard to try to obtain a second UN resolution makes it clear that the government would have preferred, for both political and legal reasons, more unequivocal support from the international community for the war.