Air passengers could have their conversations and movements monitored as work intensifies to design the terrorist-proof aeroplane.

Researchers in Britain and Europe are looking at technology that would see a comprehensive network of microphones and cameras installed throughout the aircraft, including the lavatory, which would be linked to a computer.

This computer would be "trained" to pick up suspicious behaviour, said Catherine Neary, of Bae Systems, one of the British participants in a 24 million pounds European Union project Safety of Aircraft in Future European Environment.

"It would pick passengers who are behaving oddly or in an unruly manner," she said. "They may appear nervous, or could be getting up while the plane is taxiing. If someone looks as if they are praying, the microphones would be able to tell if they were by picking up key words."

Eventually, the computer would be programmed to understand a variety of languages.

"Passengers are not being snooped on by humans, but by machines which will process the data, which would not be stored after the flight unless there is an incident," she said.

"There are likely to be cameras and microphones in the toilet, because that is where terrorists go to assemble bombs." The camera could also be trained to detect seemingly harmless items being left in aircraft lavatories that could later be assembled to make a lethal device.

"If people know they will be safer, they will be happy to accept the sensors, but we are considering the legal implications of this."

Bae Systems is co-operating with Reading University on the project designed to make the aircraft as secure as possible. "We are concentrating on onboard threat protection," said James Ferryman, a lecturer in computer science.

"We would be looking at ways in which people behave which would give rise to suspicion. It is a challenge to distinguish between situations - such as two children play fighting or someone being attacked."

The aviation industry is monitoring the project closely. ''We are always looking at new initiatives that would enhance security," a British Airways spokesman said. "BA already has CCTV which monitors activity outside the reinforced cockpit door. But we believe it is robust ground security which is the key to safety in the air."

Even before the aircraft takes off, passengers could be swept with an "electric nose" a hand-held device which could tell if they had had any contact with explosives.

Other initiatives include sophisticated biometric cameras at the check-in desk and departure gate. By comparing the iris, it could check that the passenger presenting him or herself at the airport was the one boarding the aircraft.

Work is already in hand to examine putting electronic chips on luggage that would match ones embedded in the boarding pass. They would make it easier to link passengers to their bags or, more importantly, find them when they are separated.