The stakes were raised further by a French proposal that the leaders of the security council nations should go to New York to decide between war and peace, in a vote expected next week. The White House rejected the suggestion, the British were doubtful, but the French and Russians insisted last night that their presidents were willing to make the trip.
Yesterday saw the sharpest exchange to date between the British foreign secretary, Jack Straw, and the French foreign minister, Dominique de Villepin, after France threatened to wield its veto. Mr Villepin described the British ultimatum as a "pretext for war".
In an impassioned presentation Mr Straw drew applause as he addressed Mr de Villepin and made light of his suggestion that progress in disarmament had been the result of diplomatic pressure with some help from the military build-up in the Gulf.
As he pointed at the Iraqi delegation and argued that "we put this man to the test", Mr Straw lambasted the memorandum put forward by France, Russia and Germany. "It is not even a formula for containment given Iraq's proven ability to exploit the sanctions regime to continue developing weapons of mass destruction," he said.
Mr Straw's performance made him a hero in the American delegation. By contrast, the US secretary of state, Colin Powell, delivered an address repeating old arguments made at earlier appearances, in an apparent reflection of his exhaustion with the UN process.
After another round of reports from UN weapons inspectors, which again provided an ambivalent assessment of Iraqi compliance, the security council battle lines were more sharply drawn than ever, with the US, Britain, Spain and Bulgaria backing the ultimatum, and France, Russia, China, Germany and Syria opposed. The focus is now even more intense on the remaining uncommitted six members - Angola, Cameroon, Chile, Guinea, Mexico and Pakistan. In statements made by their representatives, there were signs that intense US lobbying was making headway, but it was far from clear that Washington has the nine votes it needs to claim broad international support in the face of potential French or Russian vetos.
Britain's ultimatum came in an amendment to the draft resolution it co-sponsored with the US and Spain last month finding Iraq in breach of its UN obligations to disarm.
Under the ultimatum, Iraq would face the threat of invasion "unless, on or before 17 March 2003, the council concludes that Iraq has demonstrated full, unconditional, immediate and active cooperation in accordance with its disarmament obligations".
France rejected the ultimatum and made its clearest threat so far that it would veto such a resolution.
Mr de Villepin said: "We cannot accept an ultimatum as long as the inspectors are reporting cooperation. That would mean war.
"By imposing a deadline of a few days, would we be reduced to seeking a pretext for war? France will not allow a resolution to pass that authorises the automatic use of force."
Instead, he suggested the vote on a war resolution be taken by the leaders of the security council states in person, at a security council summit in New York.
Mr Straw portrayed the French proposal as a recipe for endless delay. "I ask the council not to allow Saddam to achieve his aim, which is to turn the final opportunity to disarm into endless opportunity for delay," he said.
Downing Street still believes it has a good chance of winning the second resolution, but concern is mounting that the US will not apply its total diplomatic weight behind the fight to secure the resolution, and that President George Bush has exhausted his patience with the UN.
The reports by the UN's chief weapons inspectors did little to break the logjam. Hans Blix noted that considerable progress had been made to secure Iraqi cooperation in recent days, but said Iraq was still withholding documents.