London will become the first city in the world to have double-decker hydrogen-powered buses next year.

Transport for London (TfL) has unveiled the newest bus to be added to its fleet: a £500,000 hydrogen model which only emits water as exhaust.

Twenty of the new buses have been ordered by TfL and will begin service on the capital’s streets next year.

Although hydrogen-fuelled single-decker buses have been running in London and cities elsewhere for years, TfL said the new double-deckers would be a world first.

As well as producing zero emissions, the buses, which are being manufactured by Northern Irish firm Wrightbus, will also include USB charging points for customers.

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The London mayor, Sadiq Khan, said: “We all have a role to play in cleaning up London’s toxic air and I’ve always said that TfL should lead from the front.

“We are investing a record £85m in cleaning up our bus fleet, and I am proud that London now has the largest zero-emission bus fleet in Europe.”

The total cost of the new buses and refuelling stations will come to £12m, although £5m of that will come from the EU.

The buses are significantly more expensive than a standard diesel double-decker, which cost about £200,000. But TfL believe the outlay is worth it to try and make a dent in London’s dirty air; 2 million Londoners currently live with illegal air pollution.

Claire Mann, TfL’s director of bus operations, said: “We know we need to go further and faster to tackle the public health emergency caused by dirty air.”

Although fully-electric buses are also being gradually introduced into some routes, the hydrogen-powered models can be refuelled much faster and have a longer range.

Last month a new 24-hour Ultra Low Emission Zone was launched, which forces older, more polluting vehicles to pay £100 a day to enter central London.

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The cleaner buses will be deployed on three routes in west London and to Wembley, and bring TfL’s zero-emission bus fleet to nearly 200 vehicles.

Mr Khan has made cleaning up London’s air one of the central aims of his mayoralty, and is starting to see results as studies show levels of some pollutants are falling.