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The planes, known as Unmanned Aerial Vehicles (UAVs), are said to be capable of unleashing lethal chemical and biological agents from aerosol devices fitted under their wings.

UK Defence Secretary George Robertson said British tornados have destroyed an airbase in southern Iraq holding more than a dozen of the unmanned planes.

"If these vehicles were fitted with some form of aerosol or crop spray to distribute, for example, anthrax they could inflict fatalities within five miles down-wind of the aircraft itself," he said.

Chief of Defence Staff General Sir Charles Guthrie said: "How far developed the programme was I am not sure. But we do know it was being worked on with great urgency and it was considered a high priority."

'Desperate measure'

However, UAV expert Nick Cook of Jane's Defence Weekly described the aircraft as a "desperate measure".

"Crop-sprayers are necessarily low-flying planes, which could easily be shot out of the sky," he said.

More alarming, he said, could be the development of a second type of unmanned plane which uses a MiG 21 fighter jet.

Robertson: UAVs could kill at a distance of five miles

The supersonic fighter, controlled by another MiG 21 in the vicinity, would be able to fly further afield and at far greater speeds than the crop-sprayer.

"We first heard they were being developed in an Unscom report about a year ago," he said.

"We then knew they were being developed, but not manufactured. It sounds like they've moved on a stage now."

However, he said even the unpiloted MiG bomber was relatively unsophisticated, as it still requires a pilot in another aircraft nearby, and has a limited range.

"It really shows how far sanctions are forcing Saddam Hussein to live off his wits and come up with such weapons," he said.

But scientists in other countries, including the UK, USA and Israel, are all working on developing pilotless reconnaissance aircraft.