The green lobby loves them, celebrities drive them, and they have become Hollywood's latest politically-correct status symbol.

But hybrid cars, which use both electric and petrol engines, still have their critics to silence.

The problem, according to campaigners, is that cyclists and blind pedestrians can't hear them coming because they are so quiet.

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Lotus is hoping specially designed speakers near the radiator will be able to successfully reproduce the traditional sound of a car engine

Which is why Lotus, famous for its gasguzzling supercars, is putting a bit of vroom back into hybrids.

Its ideas division, which works with other manufacturers, has used a Toyota Prius to synthesise the sound of a traditional engine which is then emitted through a loudspeaker mounted near the radiator.

It projects a realistic engine sound ahead of the car when it is running solely in electric mode and the sound changes depending on speed and throttle. The driver cannot hear any of the additional noise.

Concerns about an increase in accidents with electric cars seem to be supported by research. A study at the University of California found that a petrol or diesel car could be heard 36ft away but a Prius was not heard until it was 11ft from blindfolded volunteers.

Campaigners have criticised hybrid cars because they pose a safety risk to blind people who can't hear them coming

In hybrid cars, batteries charge and act as a storage device to power the electric motor, usually working when the vehicle is travelling at low speed or in traffic, therefore ideal for city driving.

The petrol engine, usually a small efficient unit, then powers the car when more power is needed such as at higher speeds.

The new engine noise system is also being developed for purely electric cars, which have also come in for criticism from blind and partially sighted people due to their almost silent operation.

Mike Kimberley, chief executive officer of Group Lotus, said: 'The increased acceptance of greener vehicles such as hybrid and electric vehicles is to be encouraged.

'They have an important role in improving fuel economy and reducing emissions.

'Our advanced external sound synthesis technology increases pedestrian safety, while retaining the car's environmental benefits.

'We hope that legislators introduce minimum noise requirements for vehicles to encourage the adoption of technologies, such as ours, which will ultimately increase pedestrian safety.'

Clive Wood, transport policy officer at the Guide Dogs for the Blind Association, said: 'Blind and partially sighted people use the noise of oncoming traffic as a cue for when it is safe to cross a road - if a 'quiet' hybrid electric vehicle is approaching then they will no longer have this cue and are immediately put at risk.'

Duncan Vernon, road safety manager for the Royal Society for the Prevention of Accidents, said: 'We welcome innovative solutions which address this.'