By CHRISTOPHER LEAKE

Last updated at 19:12 12 February 2007

Tiny cameras the size of a fingernail linked to specialist computers are to be used to monitor the behaviour of airline passengers as part of the war on terrorism.

Cameras fitted to seat-backs will record every twitch, blink, facial expression or suspicious movement before sending the data to onboard software which will check it against individual passenger profiles.

Scientists from Britain and Germany are spending £25million developing a system which they hope will make it virtually impossible to hijack an airliner by providing pilots and cabin crew with an early warning of a possible terrorist attack such as 9/11.

They say that rapid eye movements, blinking excessively, licking lips or ways of stroking hair or ears are classic symptoms of somebody trying to conceal something.

A separate microphone will hear and record even whispered remarks. Islamic suicide bombers are known to whisper texts from the Koran in the moments before they explode bombs.

The software being developed by the scientists will be so sophisticated that it will be able to take account of nervous flyers or people with a natural twitch, helping to ensure there are no false alarms.

"We're trying to develop technologies that indicate the differences between normal passengers and those who may be a threat to others, or themselves," said Catherine Neary of BAE Systems.

Mrs Neary, team leader of the Onboard Threat Detection System for the Paris-based Security Of Aircraft In The Future European Environment (SAFEE) project, added: "Blink rates come from lie-detection research and suggest the stress level is higher than normal."

The project is also developing automated flight controls that will prevent a hijacker taking over an airliner and sensors at the aircraft's doors to detect if someone is carrying explosives or chemicals.

Mrs Neary said that under the Data Protection Act, all video, audio and other recordings would be destroyed at the end of every flight so that passengers' civil liberties were not infringed.

Shami Chakrabarti, director of the human rights group Liberty, said: "Watching people constantly on aircraft and trying to work out patterns of behaviour is a difficult road to travel.

"I suspect that it will put people off flying because they will feel uncomfortable if their every blink and twitch is being monitored."

Airlines gave the scheme a cautious welcome,

indicating it would be too expensive to fit on existing commercial aircraft and that it would probably be ten years before such systems were fitted to new planes.

A British Airways spokeswoman said: "While we welcome new research and development which advances aviation security, we believe the emphasis and funding for any new initiatives would be better placed on preventing terrorists boarding aircraft in the first place.

"For example, research and development of better screening and detection equipment on the ground would be of more value at this time."