Six new eco-friendly buses are to hit the road in London, as part of the mayor's continuing attempts to reduce the city's air pollution.
The "hybrid" buses use a battery pack and electric motor, with a small diesel engine to charge it when necessary.
Running on route 360 from Wednesday, they are said to be cleaner, quieter and to use less fuel than other buses.
A pilot scheme using hydrogen-powered buses which produce nothing but steam has been extended for another year.
A London Buses spokeswoman said the steam buses, part of a Europe-wide initiative, had been successful.
But they wanted to continue to investigate other types of technology for buses on longer routes and for double-deckers.
They estimate the new buses, whose diesel engine is the same size as that of an ordinary car, will produce 89% less nitrogen oxides, 83% less carbon monoxide and use 40% less fuel.
Mayor Ken Livingstone said they were at "the forefront of environmentally-friendly technology" and would reduce emissions as well as providing a smoother, quieter ride.
They are among several of his initiatives to tackle climate change and air pollution.
After introducing a congestion charge to reduce traffic in central London, he is now considering a "low-emission zone", where high-polluting diesel vehicles would face a charge to drive into London.
Mike Weston, from London Buses, said he hoped London's bus network would lead the way in the UK and beyond in using innovative technology.
The buses' performance will be assessed over the next few months and, if successful, more may be introduced to London's fleet.
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