In London they are known as “Chelsea tractors” or 4X4s and in America as SUVs. Whatever you want to call them, the mayor of London has them in his sights. He has already called people who drive them “idiots”.

Yesterday Ken Livingstone yesterday unveiled plans to charge the highest polluting vehicles £25 a day to drive in central London in his latest revamping of the “congestion charge”. It is being seen by environmentalists as a significant advance.

Livingstone said the new charge would come into effect, subject to consultation, in 2009 or 2010, though he wants officials to speed up the timetable.

The measure would hit the owners of the heaviest 4×4 vehicles, with emissions above 225g of CO2 per km. Cars with the least harmful emissions, such as electric vehicles and hybrids such as the Prius owned by the mayor himself, would receive a 100% exemption.

According to Livingstone one in five vehicles is in the most polluting band – Band G. “It is the heaviest concentration in the country and one has to ask why people need four wheel drives in the most densely populated part of the UK. People are waking up to the fact we are bequeathing our children and our grandchildren appalling consequences if we don’t act”.

But the measure is already causing political controversy. Roger Evans, Conservative transport spokesman in London, said the aims were laudable but complained that the increased charge was punitive. “It is pushing people rather than pulling them. It is a tax and is tackling climate change in the wrong way.”

Isn’t that the same Conservative party that has a supposedly green David Cameron as its leader? Hmmm.