Mr Annan told Western leaders, including Tony Blair, he was satisfied he had reached an agreement with President Saddam Hussein on inspection of weapons sites which would not break the bottom lines set by the permanent members of the UN Security Council, including the US and Britain, before he set out on his mission.
But British ministers insisted they still needed to learn of the small print of the deal before making a final judgement. In particular, they are concerned to ensure the terms for Unscom visits to President Saddam's palaces allow as much time and as much frequency as the inspectors need.
Government sources said the deal is likely to make provision for the Secretary-General personally to authorise such visits and London and Washington will be eager to ensure this is an easily realisable formality rather than another stalling tactic by the Iraqi regime.
Mr Blair spoke to Mr Annan and to President Bill Clinton and to Robin Cook, the Foreign Secretary, who is in Brussels for a meeting of EU foreign ministers which will discuss Iraq today. Mr Cook said there had been "some encouraging signs" from Baghdad in the wake of Mr Annan's talks. "The critical issue is that Saddam Hussein must allow Unscom to have full and unrestricted access to all the sites in Iraq they want to inspect. There can be no concessions on points of fundamental principle enshrined in UN Security Council resolutions.
Mr Cook added that if the result of the talks, to be announced by Mr Annan in Baghdad this morning, is "to bring Iraq into full compliance with UN Security Council resolutions, this will be a most welcome breakthrough." Officials in Brussels with Mr Cook were saying last night that if Mr Annan had secured the hoped-for breakthrough it would be directly as a result of the clear threat of force issued by Britain and the US and would be vindication of the tough line taken by the allies despite some international opposition.
Earlier, aboard his RAF plane to Brussels, Mr Cook said that Britain had been in the lead in ensuring the mission by the UN Secretary-General went ahead. It had been essential to underline to President Saddam that the "international community meant business and was prepared to call his bluff". Mr Cook added: "I am pretty confident that Kofi Annan has left him in no doubt about that and that may be contributing to the comparative success of his visit. We won't know until Kofi Annan has fully reported how successful he has been and how far Saddam is willing to co-operate and recognise realities."
The Foreign Secretary said he "strongly resisted" press suggestions that there were differences between himself and the Prime Minister on the issue. He added: "We are both very anxious to make sure that the diplomatic track should be given every opportunity. We are both determined that military action should be seen as a last resort. We both attach the highest importance to keeping the UN on board."
Mr Cook said that since UN Resolution 687, which laid down the Unscom scrutiny of Iraqi weapons of mass destruction, was the ceasefire resolution at the end of the Gulf war, it followed that President Saddam was in breach of the ceasefire by denying Unscom its ability to work. There was, therefore, already enough legal authority to use force against President Saddam. Mr Cook said it would, however, be helpful to get another Security Council resolution, not as a "lawyer's nicety", but to make it clear beyond any doubt that the international community condemns President Saddam.
He added that if Mr Annan returned with a deal which did not meet Britain's and America's "bottom lines", President Saddam would also be in conflict with Russian and French objectives.
"I know from my conversations with the Russians that they are running out of patience with Saddam Hussein."