Just as damaging to the US position will be the insistence to the UN security council by the chief nuclear inspector, Mohamed El Baradei, that his team needs several more months to complete its work and that some important testing equipment has only just arrived in the country.

"Their report card will be a 'B'," said Mark Gwozdecky, a spokesman for the International Atomic Energy Agency (IAEA), which is carrying out the nuclear inspections. "We've been getting where and when we want to get, and we've been generally successful in getting what we need."

Another IAEA official, Melissa Fleming, said that 16 soil samples analysed for radiation had so far proved negative, but added that there were more samples to be taken and that equipment to test for airborne gamma radiation had only just arrived in Iraq.

The IAEA's assessment in effect knocks away half of the platform upon which the US is hoping to build its case against Saddam Hussein when the security council meets on Monday. The other half was also looking shaky yesterday.

Hans Blix, who is in charge of biological, chemical and missile inspections and who will also present a report, has also called for more time. According to one well-placed UN official, Mr Blix's inspectors are demanding "several more months" to pursue their operations in Iraq.

"Nobody can imagine the inspectors could do a proper job in eight weeks," the official said.

However, Mr Blix has been ambivalent about Iraqi cooperation, pointing out Baghdad's objection to his inspectors using American-piloted U2 spy planes to search Iraq for banned weapons. Mr Blix is also unhappy with the refusal of Iraqi scientists to undergo interviews without a government minder present.

The Pentagon has alleged that the scientists and their families are being threatened with execution if they cooperate with the inspectors. Yesterday, the White House spokesman said it was "unacceptable" that the scientists had not been made available for private interview.

In the battle for public perceptions, the Bush administration has been conducting an intense media campaign in an attempt to ensure the headlines from Monday's crucial reports by the weapons inspectors will portray Iraq as having failed to disarm.

If the coverage emphasises Baghdad's cooperation and the absence of a "smoking gun" the administration will have an even harder task convincing the rest of the world and the US public that the inspections should be cut short to pave the way for war.

Nearly 70% of Americans questioned recently said the inspectors should have months to finish their work.

"Is the glass half full or half empty. That will be the question on Monday," said a diplomat from one of the security council member nations.

An administration official said yesterday that if the inspectors produce new evidence of clandestine Iraqi attempts to produce weapons of mass destruction, the administration would consider giving them more time. The implication was that if the inspectors have little to declare on Monday, the US will reject their work as pointless.

In that case, the Bush administration will be forced to rely on presenting its own intelligence to justify going to war. John Bolton, US undersecretary of state - and a leading hawk - said Washington had "very convincing" evidence of an extensive Iraqi programme for the production of banned weapons which it will reveal "at an appropriate time".

The evidence includes long-range missiles which Iraq has been banned from keeping since the 1991 Gulf war, he said.

The previous day, the deputy defence minister, Paul Wolfowitz, said that Iraq had admitted trying to produce rocket fuel of a type suitable for ballistic missiles. But analysts said the US case would have to be more convincing to change minds in Europe and much of the US.

"If the Americans wanted this to be the linchpin for building the coalition, it would have to be very conclusive evidence, such as photographs," said Toby Dodge, an Iraq specialist at Warwick University. "Either this is a well-timed grand finale, or they don't have anything. My first instinct is that if the US had conclusive evidence we'd see it now."

An Iraqi diplomat also highlighted Washington's coyness about disclosure. "What do they mean by an 'appropriate time'? What is a more appropriate time than while the inspectors are preparing their report," he asked.