Advice on the legality of a war with Iraq, Lord Goldmsith, Attorney General for England and Wales, to UK Prime Minister Tony Blair, March 7, 2003 [released by the PMO April 28, 2005; indicating that a war could be considered legal for the United Kingdom, but also saying in light of reservations that the safest course would be to obtain a second UN Security Council resolution directly authorizing the use of force]. Excerpt:

To sum up, the language of resolution 1441 leaves the position unclear and the statements made on adoption of the resolution suggest that there were differences of view within the Council as to the legal effect of the resolution.

Arguments can be made on both sides.

A key question is whether there is in truth a need for an assessment of whether Iraq's conduct constitutes a failure to take the final opportunity or has constituted a failure fully to cooperate within the meaning of OP4 such that the basis of the cease-fire is destroyed. If an assessment is needed of that situation, it would be for the Council to make it.

A narrow textual reading of the resolution suggests that sort of assessment is not needed, because the Council has predetermined the issue. Public statements, on the other hand, say otherwise.

27. In these circumstances, I remain of the opinion that the safest legal course would be to secure the adoption of a further resolution to authorise the use of force. […]

Read the full text of the advice [PDF]. Reported in JURIST's Paper Chase here.