The most well-known hybrid car, the Toyota Prius, is to be fitted with a noisemaker after concerns have been raised that it risks the lives of pedestrians who are unable to hear it coming.

Hybrid cars operate on electric power when travelling slowly and switch to traditional petrol when going at faster speeds. When moving slowly, the Prius is currently barely audible.

The Prius is Toyota's top-selling hybrid car and has long been praised for its green credentials by motoring organisations. But a US National Highway Safety Administration study found that hybrid vehicles were twice as likely to be involved in road incidents with pedestrians as those using petrol engines – so the decision has been taken to fit it with a noisemaker.

A spokesman for Toyota in the UK said, "It's a humming sound, a bit like the space-age cars in The Jetsons."

The noise generator will take effect when the car is operating on electric power at a low speed and speakers on the bonnet will cause the pitch of the sound to change depending on the rate at which the vehicle is moving.

The Prius noisemaker will be launched first in Japan on 30 August at a cost of £97 plus installation costs. It is already the country's most popular car.

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