Mr Annan was due to fly from New York to Paris, where he will hold talks this evening with President Jacques Chirac who has been trying in vain to broker a settlement of his own. But France, though opposed to the use of force, has now warned Iraq that unless it complies with the UN weapons inspectors, it faces certain air attack from British and American forces.
From Paris, the Secretary-General flies on to Baghdad where he will stay until Sunday. This means that any onslaught will not start until Sunday night at the earliest. It could be delayed even longer, since the United States Congress is not scheduled to vote until Tuesday on a resolution approving the use of force against Baghdad. But there is no guarantee that President Bill Clinton will wait that long, if Mr Annan's attempt has plainly failed.
And he will have precious little room for manoeuvre. Although Mr Annan himself declared his chances of success to be "reasonable" and that the Iraqis seemed ready "to engage me constructively", the US ambassador to the UN, Bill Rich-ardson, made it clear that US patience with Baghdad's evasions and backsliding had run out.
In the Commons, Tony Blair was equally uncompromising. The success of the Annan trip depended entirely on Saddam Hussein, the Prime Minister declared, leaving no doubt of the view of Britain and the US that the Secretary-General was going with what amounted to a take-it or leave-it offer.
All substantial concessions, in the view of London and Washington, will have to be made by the Iraqi leader, and that Mr Annan will not be going for a bargaining session. He, himself, summed up his mandate as one of being "firm in substance and flexible in form".
Closer to ground zero of any assault meanwhile, tensions mounted and precautions gathered speed. As students in Egypt and Jordan marched against an attack, the UN announced that it was sending 31 members of its staff in Iraq out of the country. For their part, the inspectors whose search for Saddam's chemical and biological weapons plants sparked the latest crisis, completed their visits to eight "presidential compounds".
A formula for UN access to these sites could provide an 11th-hour diplomatic solution to the crisis, and the Egyptian President, Hosni Mubarak, who contacted both Mr Clinton and Saddam Hussein yesterday, claimed to detect an Iraqi "understanding of the situation".
But London and Washington are insisting that no serious strings can be attached to an "Unscom-plus" scheme, whereby the inspectors would go over the plants with a toothcomb, but accompanied by other officials or diplomats. The team had to have full and unfettered access to the sites. "Our resolve on that is right and immovable," the Prime Minister told MPs.
Earlier the Secretary of State for Defence, George Robertson, was blunter still, vowing that whatever expert doubts on the issue, air strikes would inflict "massive damage" on President Saddam's facilities. Nor would "human shields", as employed by the Iraqi leader during the Desert Storm bombings in1991, be a deterrence. "We cannot account for Saddam's uncivilised and quite illegal behaviour" Mr Robertson said.
Meanwhile, Mr Cook last night said that a new UN resolution would be necessary to authorize an attack. It might be better from a legal and political point of view to update existing resolutions, but it was not "necessary".