But as American and British forces continue their build-up in the Gulf ahead of a possible war, diplomats from both countries insist that the Iraqi regime has not yet crossed the line that makes war inevitable.

After weeks of lengthy negotiations in the autumn, the 15 members of the United Nations Security Council set out clearly what would constitute Iraq’s failure to comply with the resolution described as Iraq’s “final opportunity”.

Paragraph four of UN Security Council Resolution 1441, passed unanimously on November 8, stated that: “False statements or omissions in the declarations submitted by Iraq pursuant to this resolution and failure by Iraq at any time to comply with, and co-operate fully in the implementation of, this resolution shall constitute a further material breach of Iraq’s obligations and will be reported to the Council.”

The key word in the paragraph is “and”, meaning that simply failing to make a full declaration in itself would not constitute the “material breach” necessary to justify the “serious consequences”.

The diplomatic euphemisms mean, essentially, that Iraq has to lie in its declaration and also obstruct the work of inspectors before Washington and London can use force against Baghdad.

“You may hear the Americans use the word ‘material breach’ in the coming days, but it will probably be used only in a rhetorical sense,” one Western diplomat at the UN said. “As far as the other 14 members of the Security Council are concerned, lying or making omissions from the declaration is not a trigger in itself, it is not a casus belli. The Americans know that as well as anyone — they wrote the document.”

While British forces are being made ready for action, Foreign Office officials say that they will not be used until Saddam is in clear breach of his international obligations.

“You cannot prove that they are lying or have made an omission until their declaration is tested,” one official said. “Either the inspectors will find suspect materials, or they will have the doors slammed in their faces when they attempt to enter a building, or interviews with Iraqi scientists and officials are prevented.”

The Americans are likely to step up the pressure in two ways. First, they will press the UN to be more forceful in its inspections and may supply specially selected inspectors with sensitive intelligence on the location of hidden nuclear, chemical, biological materials or ballistic missile technology.

The other route is to interview Iraqi scientists known to have been involved in producing the country’s weapons of mass destruction. Last week, Hans Blix, the chief UN weapons inspector, wrote to Iraq demanding the names of scientists involved in the weapons programmes, which Iraq is duty-bound to divulge under the terms of the UN resolution.

Assuming that Iraq complies, the UN is then expected to begin interviewing the Iraqi scientists, who may be the only people besides Saddam and his closest advisers who know what weapons Iraq has and where it is hiding them.

As the UN efforts are stepped up, so, too, will be the pressure exerted on Saddam militarily and politically. With tens of thousands of American troops massing in the Gulf and British forces also being mobilised, Washington is hoping that the pressure could trigger a coup against Saddam.

Another deliberate challenge to his authority is being prepared for mid-January, when the leadership of the Iraqi opposition is to hold a conference in the northern city of Arbil to discuss Iraq’s future post-Saddam. The city, in the autonomous Kurdish region protected by US and British warplanes patrolling the northern no-fly zone, is only about 200 miles north of Baghdad. A conference held there at this critical time would be an enormous challenge to Saddam’s authority.

“Saddam’s past history suggests that as the pressure is built up around him he will eventually slip up and give the Americans the excuse they are looking for,” one expert on Iraq said.

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