The Inscentinel machines could be installed at airports and border posts to intercept potentially lethal terrorist cargoes and packages. Professor Paul Davis, chief executive of the Hertfordshire based company, Insense Ltd., said security services around the world have already expressed interest in the new technology.

"Bees have a remarkable sense of smell and are able to pick up odours from explosives such as TNT very easily," he said. "Our system allows us to harness that natural ability."

The system is designed to be more reliable than sniffer dogs, which have a habit of getting distracted, he added. Insense has built prototypes of the detectors, which are the size of a shoebox and contain about 20 bees in a honeycomb interior.

Each of the insects - which normally spend their lives flying about finding pollen for the hive - has been conditioned to respond to a certain smell. If the bee detects the target odour, such as an explosive, it extends its long tongue - a movement which is instantly picked up by inbuilt computers. The system user is then alerted to the potential threat. The bees are housed in removable cassettes and spend just two days of their four-week-long lives inside the detectors. They are then sent back to Insense and allowed to live the rest of their time in freedom. Professor Davis said: "bees have this reflex reaction to particular smells. Normally, they respond to flowers but they can easily be conditioned to respond to any smell if we reward them with food. They can detect just about any biochemical molecules in the air, whether that be from TNT, tobacco or drugs.

Typically, we have three bees working together to detect one particular smell. If they all respond positively, we can have extra confidence that the target has been detected. It is not cruel to them at all. They are able to fly around at leisure and the machine feeds them automatically. It is a bit of a holiday for them, actually."

Researchers at Insense have been working on the device for two years. But they say it is likely to be another two years before the detectors can be mass produced. Professor Davis says the technology is "just the tip of the iceberg in terms of new technologies that use natural systems to detect odours."

Similar systems could be used to find landmines, many of which are undetectable by conventional means because they contain no metal parts. The devices could even be used by doctors to diagnose illness by detecting the "smell signature" of certain diseases.