Manufacturers are working on fitting an artificial engine sound

The world's first purpose-built hydrogen-powered bike could be fitted with an artificial "vroom" because of worries its silence might be dangerous.

A prototype of the motorbike, which could cost more than $8,300 (£4,500), was unveiled in London on Tuesday.

The problem with the "fuel cell" bike, which produces no polluting emissions, is that it is too quiet.

But anti-noise campaigners said they welcomed the prospect of a motorbike without the usual excruciating roar.

For their part, manufacturers said the fake engine noise device, which could be switched off, would help alert road users.

The motorbike, known as an Emissions Neutral Vehicle (ENV), has a top speed of 50mph (80km/h), a range of at least 100 miles (160km) and can run continuously for four hours before the fuel cell needs recharging.

Its water-vapour emissions are so clean that they are drinkable, according to its designers.

Mobile energy source

But with a noise emission equivalent to an everyday home computer, motorcycle enthusiasts thought the "exhilaration" factor was missing.

"They can add all the noise they want, it will still lack the va-va-voom serious motorcyclists look for," Jeff Stone of the British Motorcyclists Federation told the BBC.

Concerns were raised that the motorcycle was too silent and might not be noticed by other traffic and pedestrians.

Harry Bradbury, chief executive of the bike's British manufacturers Intelligent Energy, said: "What we are doing is introducing flexibility into it, so that you can have ambient noise that is tolerable - low-level noise sufficient for safety reasons - but which can be switched off when desired."

Peter Wakeham, director of the Noise Abatement Society, who said motorbikes were among the worst noise offenders, welcomed the idea of a quiet bike.

"But it kind of defeats the purpose of designing a silent bike only to then add an artificial noise device," he said.

Dr Bradbury said the bike's detachable briefcase-size cell filled with high pressure hydrogen, or "core", could eventually be used as a mobile energy source, with the same cell used to power different objects.