The advice offered by Britain's top legal adviser on the legality of the war in Iraq has exposed differences of opinion with the US.
There was much discussion in Britain on the war's legality
UK Attorney General Lord Goldsmith concluded that the US doctrine of using force to pre-empt future danger was not recognised by international law.
The government had previously withheld the advice, but fresh media leaks this week prompted its publication.
The document highlights the diverging views, glossed over at the time.
Lord Goldsmith's legal advice was prepared for British Prime Minister Tony Blair on 7 March 2003, as the momentum towards war was building.
The 13-page document - marked "secret" at the top of each page - considers mainly whether war could legally be engaged on the basis of previous resolutions agreed by the UN Security Council.
Lord Goldsmith said he was not sure it could.
He also dismisses arguments used by the US to justify war.
"I am aware that the USA has been arguing for recognition of a broad doctrine of a right to use force to pre-empt danger in the future," the document says.
"If this means more than a right to respond proportionately to an imminent attack (and I understand that the doctrine is intended to carry that connotation), this is not a doctrine which, in my opinion, exists or is recognised in international law."
On regime change, Lord Goldsmith said Saddam Hussein could be removed as long as "such action is a necessary and proportionate measure to secure the disarmament of Iraq".
"But regime change cannot be the objective of military action."
The different justifications for war adopted by the US and UK were noted at the time, but have never before been so glaringly exposed, correspondents say.
The legal advice shows the chief legal adviser in the United States' closest ally explicitly disagreeing with the US approach, they add.