The US west coast city of San Francisco and Sweden's capital Stockholm are jumping on the bandwagon set in motion by London Mayor Ken Livingstone in 2003. French cities, including the capital Paris, are now studying such a charge.

A spokesman for Livingstone's office told AFP that other US and European cities, including one in Italy, and "some cities in Asia" have sent delegates to London to explore the idea, though he declined to name them.

The charge proved highly unpopular at first among drivers, merchants and others, but Livingstone has survived the storm and won the admiration of ecologists.

"Greenpeace very much supports the work of Ken Livingstone in the way that he has taken some quite bold steps to tackle climate change," according to Emily Armistead, Greenpeace's climate campaigner.

"We think it's excellent that other cities might now follow his lead," Armistead told AFP.

When the levy was introduced in 2003, motorists paid five pounds (9.50 dollars, 7.50 euros) a day from Monday to Friday to drive in the City of London financial district and the West End, the city's main commercial and entertainment center.

Drivers now pay eight pounds a day, and the scheme will be expanded westward in February to include the plush neighborhoods of Knightsbridge, Kensington and Chelsea.

Armistead hailed Livingstone's plans, announced last Tuesday, to triple the daily congestion charge to 25 pounds for large cars and trucks in the city and exempt cleaner vehicles.

Subject to legal review, the levy could be introduced in 2009, or 2010.

Armistead said that imposing higher fees on gas-guzzling four-wheel drive vehicles than on smaller cars, while exempting electric cars, will force consumers to make choices about the cars they buy.

"That in turn can have a real effect on the market and push car companies themselves into manufacturing cars that pollute less and produces less carbon dioxide," she said.

Scientists say carbon dioxide contributes to global warming by trapping heat inside the earth's atmosphere.

Tony Bosworth of environmental campaigners Friends of The Earth said Livingstone's move to target gas guzzlers complemented proposals in the southwest London borough of Richmond to introduce different rates of parking tax based on the level of each car's emissions.

"There are lots of different ways in which we can encourage people to buy and use more fuel-efficient cars which is vital in the fight against climate change," he said.

It will also help, he said, once Britain begins charging cars for distances traveled on the nation's roads -- a plan that might take a decade to implement.

Bosworth warned that cities adopting congestion charges must also invest simultaneously in improved public transport, creating safer streets for cycling and walking, as well as better land use planning.

He hailed the congestion charge first of all as a practical success, even if traffic levels were still rising in outer London.

"It's cut congestion levels, it's cut traffic levels, it's increased public transport and it's increased cycle use in central London," Bosworth told AFP.

The mayor's office said that, compared to 2002, traffic congestion is 22 percent lower, carbon emissions have decreased 16 percent, nitrogen oxide pollution has declined 13 percent and particulate matter is down by 15 percent.

Livingstone has proven that unpopular measures can eventually be widely accepted.

"It can first seem like a sacrifice, but people see real benefits such as better quality of life," Armistead said.

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