easyJet

The Unite union said legal notice has been served in 51 cases, the majority of which are against British Airways.

EasyJet, Thomas Cook, Jet2 and Virgin Atlantic are also subject to the legal action over "aerotoxic syndrome".

The airlines said that previous studies found no proof of long-term ill-health arising from cabin air quality.

The Unite union, which represents airline staff, claims pilots and crew are exposed to frequent "fume events" when air drawn into the aircraft becomes contaminated by toxic compounds.

The eco-friendly aircraft has a hydrogen fuel cell at its core and energy-absorbing brakes that draw in reusable power on landing.

The energy-absorbing brakes and hybrid engines are designed to reduce fuel consumption and carbon emissions, plus having less fuel to carry around should reduce costs for airlines.

eastJet's engineering director Ian Davies came up with the concept and enlisted the help of students at Cranfield University to put the prototype designs together.