Ken Livingstone, the Mayor of London, provoked a storm of protest from motorists and business groups today with a plan to slap a £25 congestion charge on gas-guzzling "Chelsea tractors" to cut carbon emissions in the capital.

Mr Livingstone said that he was asking Transport for London to draw up proposals to amend the congestion charge scheme to take emissions into account.

He said that he was in favour of lower charges for greener cars but suggested that the £8 charge for entering Central London could trebled or "environmentally damaging vehicles" such as 4x4s.

"The congestion charge has been a huge success in reducing traffic levels and supporting the public transport system through the revenue raised and improvement to bus reliability," Mr Livingstone said.

"I now want Transport for London to bring forward plans to build on the success of congestion charging to encourage drivers in London to purchase a low-emission vehicle. My aim is to see the cutting of carbon emissions and the protection of our environment at the heart of the scheme.

"There is a growing sense of concern amongst Londoners about climate change caused by CO2 emissions, which is the biggest single problem facing humanity, and tackling this threat requires decisive action.

"I want TfL to look at lower congestion charges for cars responsible for lower than average CO2 emissions, broadly the retention of the current rate of £8 for most cars, and much steeper charges of perhaps £25 for the really environmentally damaging cars such as the so-called ‘Chelsea tractors’.

"This is a charter for greening our car choices and it will once again put London at the cutting edge of both environmental and transport policy."

The Labour Mayor's latest campaign might prove to be well-judged politically given the opprobrium faced by drivers of 4x4s in urban areas, especially mothers on the school run.

The C-charge zone already includes some of London's most affluent residential areas, including Mayfair, and there are plans to extend it westwards so that it would include Chelsea itself - where having at least one 4x4 in the garage is considered de rigueur for the smart set.

Angie Bray, the London Assembly Conservative Leader, said that that the move could leave families stranded and damage small business.

"Cutting pollution is a good thing but this change will hit small businesses and families, both of whom have to use larger vehicles because of their circumstances" she said.

"Mr Livingstone is now shifting the focus away from cutting congestion and on to raising money for the Mayor’s other congestion-causing transport projects. Reducing the charge during the period when the roads are quiet would be a much better way to cut congestion and therefore emissions.

"We all want cleaner air in London but this could be a very painful way forward. I hope that for once Mr Livingstone’s offer to consult London is genuine."

Paul Watters, head of roads and transport policy at the AA Motoring Trust, said: "A focus on so-called gas-guzzlers smacks of jumping on a bandwagon of prejudice. The hardest hit are often drivers who are trying simply to go about their lives in the most efficient and cost-effective way according to their circumstances.

"The war on 4x4s in London has reached new heights and is in danger of going over the top by introducing punitive daily charges for many other vehicles that are nothing like the vehicles being targeted.

"It has to be realised that they already pay a recently introduced top rate road tax, and probably pay more fuel duty and VAT as they use more fuel."

Damian Hockney, leader of the One London Party, which has two members on the London Assembly, said: "The Mayor is in deep trouble with the congestion charge and, despite his denials, this is a desperate bid to boost c-charge revenue, using cod science as a feeble excuse.

"The Mayor has announced another sham consultation, but from his announcement it is clear that his mind is already made up. Business has already been hit by the c-charge. The westward extension is another disaster in the offing and this latest shock is yet another blow to central London. This will be a major election issue in 2008."

But environmentalists welcomed Mr Livingstone's proposals. Sian Berry, of the Alliance Against Urban 4x4s, said: "We are delighted that the Mayor has taken on the problems caused by big, gas-guzzling cars in our city with so much enthusiasm.

"There is simply no place for these monsters in our cities and the evidence shows that financial penalties, like a £25 congestion charge, will make a real difference. Our report to the Mayor last month demonstrated that a higher charge could deter up to 40 per cent of 4x4s from coming into Central London."

Charlie Kronick, head of Greenpeace’s climate and energy campaign, said: "By discouraging Londoners from unnecessarily driving gas-guzzling 4x4s around the city, the Mayor’s proposals will help slash global warming emissions and make London a safer place to live and work."

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