The vehicles will run on tanks of gas stored in their bulletproof boots.

Police chases with roaring engines beloved of fans of 1970s police television series such as The Sweeney will become a thing of the past as the new “H-cars” run silently. Their exhausts will leave nothing more harmful than a trickle of water.

The car is still under development by Honda but the Metropolitan police plans to take four of the new cars from a delivery of 70 due to go the Greater London Authority by 2010.

The pioneering move to buy the hydrogen cars is part of a drive by the Metropolitan force to cut emissions of greenhouse gases as conflict in the Middle East and volatility in the oil markets has sent conventional fuel prices soaring at the pumps.

The Met already has 90 cars in its fleet of 6,000 vehicles which use a hybrid fuel, mixing diesel and vegetable oil, and it is to take delivery of another 128. It has also experimented with battery-powered electric cars and others run on liquefied petroleum gas (LPG) but these have been returned to the manufacturers.

The Honda FCX hydrogen car is a new development, however. The gas is stored in a secure tank in a boot capable of withstanding gunfire and crashes.

The gas is fed under electronic control to a fuel cell where it mixes with oxygen in the air intake to create electricity and water.

The electricity created runs a drive unit under the bonnet. The water is piped away as vapour via the exhaust system.

A Scotland Yard spokesman said: “We have to wait for the specifications to see what we can do with them. They may be used as patrol cars where the traffic flow makes sense. Nothing goes very fast in Trafalgar Square.”

An American motoring writer who drove a test model said the vehicle felt more like a tram than a car, “smooth and steady, without the jolt of shifting gears.

“One oddity is a ghostly warble that intensifies as I speed up, like bad sound effects from a Scooby-Doo cartoon.”

Other manufacturers are working on their own hydrogen cars. The first to go on active police duty is a DaimlerChrysler that patrols a university campus in Detroit. BMW, which makes Scotland Yard’s fast pursuit cars, said it hoped to have a high-performance model in operation by 2009.

Bruce Reynolds, mastermind of the 1963 Great Train Robbery, bemoaned the passing of the golden age when London streets were empty enough for car chases.

“I look at these things much the same as an old soldier might look at campaigns,” he said.

“The hydrogen cars are still being developed, but obviously the police of today are highly technical and move with the times. We had the better drivers in my day as a criminal but the police have always had access to vehicles that are not strictly on the agenda.”

Roy Ramm, a former head of the Flying Squad, said: “The Met are looking for low-end rather than high performance with these new cars. It will mean huge savings.

“Judging by the hot air that comes out of Scotland Yard these days, they will be able to refuel them themselves.”

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