LONDON (AFP) - A new hydrogen car designed for use in cities and backed by a relative of the founder of German luxury sportscar maker Porsche was on Tuesday unveiled in London.

The two-seater Riversimple Urban Car can travel 240 miles (386km) without refuelling, weighs just 350 kilogrammes (770 pounds) and has a top speed of 50 miles per hour.

In fuel efficiency terms, it can do the petrol equivalent of 300 miles per gallon (0.78 litres/100km) and emits only water. Manufacturers hope it could go into production from 2013.

The cars will be leased to owners for a reported 200 pounds (S$476) per month rather than sold, as part of a package including maintenance, support and fuel.

The project involves Sebastian Piech, a great-grandson of Ferdinand Porsche, who founded the famous German car company in the 1930s.

Piech said the car represented a "major step... towards the fulfilment of my great-grandfather's ambitions for accessible personal transport but this time combining his other passions: light weight and high efficiency".

He is executive director of a firm which works on hydrogen fuel cells which has helped to develop the cars over the last three years.