Microphones will eavesdrop on passengers’ conversations while computerised CCTV detects suspicious movements so that hijackers can be caught before they go into action.

The plans, being developed by a consortium including BAE Systems, Airbus and the European commission, are designed to counter terrorists who succeed in slipping through airport security.

The systems, which have been in development since Al-Qaeda’s attacks on America on September 11, 2001, go far beyond the reinforced cockpit doors and sky marshals introduced as an immediate reaction to the terror strikes.

The first tests were carried out last month using actors in planes on the ground in Bristol and Hamburg.

“We cannot say you will reach a zero level of risk, there is no such thing, but we think it is important that the public is reassured we are doing everything possible to get them maximum protection,” said Daniel Gaultier, the Paris-based co-ordinator of the project, named Security of Aircraft in the Future European Environment.

The first elements of the technology will be available to airlines by mid-2008 and the full system should be installed a few years after that. Similar work is being done separately in America.

The system is designed as a “last-ditch” defence if airport checks and passenger surveillance fail to prevent a hijack.

New cockpit doors, in addition to being strengthened, are likely to be controlled using biometric technology — which scans irises and fingerprints — so only authorised crew can gain access.

There will also be biometric sensors fitted to cockpit instruments so that if — as in the 9/11 attacks — a terrorist kills the pilot, the plane’s controls can be overridden.

A computer would then prevent the plane being taken off its pre-determined course and allow ground controllers, by remote control, to land the aircraft safely at a nearby airport.

As a last resort — for example if the terrorist holds a gun to the pilot’s head and tries to force him to fly into a building — an emergency avoidance system is being designed that would mean the plane automatically changed course if it headed for buildings or mountains.

Instruments would be able to judge whether the plane was being steered towards a target and would override the pilot and direct it away towards clear sky.

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