Powered by a high pressure hydrogen fuel cell, the Emissions Neutral Vehicle (ENV) produces the equivalent noise of a personal computer fan belt.

Not only is that distinctly wimpish in the eyes of many bikers, it could also be dangerous.

Makers Intelligent Energy are looking at ways to produce an artificial engine noise that will alert people to its presence, making sure the machine is not silent and deadly.

"We will consider that," said Nick Talbot, the project leader at Seymourpowell, who were hired by Intelligent Energy to design a bike to their brief.

The British designed and built bike, which has no gears, can reach speeds of 50 miles per hour.

Motorcycle enthusiasts have welcomed the green innovation but say some bikers like the roar of an engine and the thrill of going fast.

"It fits the definition of a motorcycle, but not as we know it," said Jeff Stone, a spokesman for the British Motorcycle Federation.

"The motorcycle is a primitive thing and it appeals to the inner person. The excitement and exhilaration of a bike is why people ride them."

Stone, however, believes that the "soft and cuddly" green bike could be useful in city traffic.

The bike's briefcase-sized fuel cell needs to be topped up every 100 miles and so far there is only one station in Britain that supplies the type of hydrogen required.

But its makers believe that will change.