Hydrogen fuel cells (HFCs) produce energy by oxidising hydrogen into water, giving water vapour as the only exhaust gas. Extra energy is needed to obtain the hydrogen in the first place, but even if this comes from natural gas the overall result is a major reduction in emissions.

In the long run, HFC technology could help combat air pollution and CO2 emissions from urban transport. But in a city like London, where 6,800 buses carry six million passengers a day, the main role of London's seventy HFC buses will be to push for technological progress.

All major vehicle manufacturers are already developing hydrogen-run models, but London's Mayor hopes the initiative will help speed up the process.

"Hydrogen fuel cells could offer a real alternative to diesel in the future," mayor of London Ken Livingstone said.

"The high cost of the vehicles is the major barrier at the moment but the greater the demand for vehicles, the more the costs will come down. I would call on the manufacturers to gear up for this change, as hydrogen vehicles are a real and viable option for London."

The HFC bus fleet expansion is part of the London Hydrogen Partnership, a wider drive towards a "hydrogen economy" for London.

Deputy Mayor Nicky Gavron, Chair of the London Hydrogen Partnership, said: "I am pleased that the Mayor has agreed to support the London Hydrogen Partnership's ambitious transport programme."

"This will move London to a position of leadership on this issue in Europe and around the world, and sends a strong signal to industry that London is the place to come to deliver hydrogen vehicles. The London Hydrogen Partnership is looking to join up with other cities and regions in the world to speed up development of this technology."

Three London buses already run on HFCs, served by a hydrogen refuelling station in Hornchurch. Transport for London is in the process of acquiring another ten, with sixty more to follow by 2010.