Britain and the US were furious that Dr Blix did not present what they regard as damning evidence and insisted that he attend yesterday's security council meeting to answer questions on the drone, cluster bombs and other outstanding issues.

But Dr Blix said yesterday that just because the Iraqis had not declared the drone did not make it a smoking gun and promised to publish the key remaining disarmament tasks that Iraq should perform some time next week.

The drone, with a 24ft wingspan which could allow it to fly further than the 150km UN limit, was included in an annexe to the UN monitoring, verification and inspection commission (Unmovic) working document that was added at the last minute.

"Nowhere in the document is it asserted that Iraq has weapons of mass destruction," he said. But the crucial issue where the drone was concerned depended on its range and its function, he said.

"Iraq should have declared this vehicle," he said. But he went on to insist that the crucial factors where the drones were concerned depended on both its range and its function. "The legality [hinges on] if it has a reach longer than 150km and whether we can establish if it is linked to biological and chemical weapons."

According to the working document, the inspectors found part of a bomblet designed for chemical and biological agents last month in the Al Noaman munitions factory.

"Iraq stated that this was a leftover from the past declared chemical simulant test programme that was abandoned," the report notes, but it adds that the evidence suggests that Iraq may have made more progress with cluster munitions than it had declared.

The working document points out that earlier inspections of a site known as Haidar Farm, had unearthed documents referring to a cluster weapons programme known as "Project 101", but said that Iraqi officials had denied any knowledge of such a project at a meeting with Unmovic inspectors last month.

US officials were angry that Dr Blix failed to bring up the discovery of the new form of drone aircraft last week.

The US state department is particularly incensed because this appears to confirm a claim the secretary of state, Colin Powell, made in a presentation of US evidence to the security council on February 5. Dr Blix has referred to this, but principally to question its claims.

Dr Blix last night dismissed the criticisms as a reflection of the pressure his inspectors are under within a deeply divided council.