In a speech to be delivered shortly before George Bush addresses the UN general assembly, Mr Annan will declare that the Iraq crisis brought the UN to a "fork in the road" as decisive as 1945 when the world body was formally established.

The 191 UN member are struggling to heal deep rifts caused by the war on Iraq, in which the US acted without the aproval of the security council.

In a text of his speech released in advance, Mr Annan questions US arguments that nations have the "right and obligation to use force pre-emptively" against unconventional weapons systems, even while they are still being developed.

"My concern is that, if it were to be adopted, it could set precedents that resulted in a proliferation of the unilateral and lawless use of force, with or without credible justification," he says.

He says the UN charter allows military action for the purpose of self defence.

"But until now it has been understood that when states go beyond that and decide to use force to deal with broader threats to international peace and security, they need the unique legitimacy provided by the United Nations.

"Now some say this understanding is no longer tenable since an 'armed attack' with weapons of mass destruction could be launched at any time.

"This logic represents a fundamental challenge to the principles on which, however imperfectly, world peace and stability have rested for the last 58 years."

President Bush plans to make a defiant stand at today's UN meeting, demanding international support in cash and troops for the US occupation of Iraq, while rejecting a speedy transfer of authority to Iraqis.

Excerpts of his speech to the general assembly released yesterday make it clear that there remains a gulf between the US and its European critics, led by France, about the governance of Iraq.

But both sides are clearly anxious to avoid the bitterness of earlier UN debates on Iraq, offering goodwill concessions.

France promised not to brandish its security council veto, while Mr Bush said the UN could help draw up a constitution - "they're good at that" he told Fox News - and play a role in supervising eventual elections.

The timetable for those elections and the transition to Iraqi self-rule is at the heart of the current debate, in which the US is insisting on a deliberate pace, going deep into next year, and the French are calling for a much faster handover.

Condoleezza Rice, Mr Bush's security adviser, said last night that the transfer must follow "an orderly process." She told reporters: "The French plan, which would somehow transfer sovereignty to an unelected group of people, just isn't workable."

According to one version of the text yesterday, Mr Bush will offer a compromise solution, asking the Iraqi governing council to come up with a schedule for elections.

Most of the council's 25 members are in favour of a fast-paced transfer and several are in New York to lobby for that option. But a diplomat in New York said it was unlikely that the US would surrender the decision to the council without a prior agreement that it would respect Washington's wishes.

Mr Bush will urge UN members to bury their differences on Iraq, send money and soldiers, and focus on other global problems, such as nuclear proliferation in "rogue states", Aids and human trafficking .

President Jacques Chirac, who will address the assembly after Mr Bush, indicated that France would not veto a US-backed security council resolution calling for military and financial support for Iraqi reconstruction, but also made clear that Paris would not answer that call unless authority was handed over to Iraqis promptly.