Although few outside the retail trade and motoring lobby want it scrapped, more Londoners are questioning its form and scale, fearing it has become a tax, rather than a congestion beater.

Last week, Mayor Ken Livingstone, fighting for a third term in May's election, announced a populist new add-on to the charge - a GBP25 (HK$380) per day levy for drivers of the most polluting cars (think sport utility vehicles, limos, and some sports cars and people carriers); some GBP6,500 a year.

The charge, to take effect on October 27, follows the extension of the C-charge zone into central west London last year, and a rise to GBP8 a day from the GBP5 imposed in February 2003. Since then, 70,000 cars a day have been taken off central London roads, although critics claim the cars still exist but are just being driven on the periphery.

The scheme's evolution has met with an increasingly shrill chorus of opposition. Critics say the tariff on SUVs and sports cars has transformed the charge from an anti-congestion measure into an environmental stealth tax.

Brian Paddick, the Liberal Democrat candidate for mayor, claims the tax will not affect the well-off but will hurt 'hard-working families with people carriers who cannot afford to change their car or pay the charge'.

The Conservative candidate, Boris Johnson, branded the move an old-style car tax. 'Londoners use their cars because of the appalling state of the transport system. A big car tax won't change that.'

Around 30,000 vehicles - about a fifth of the 150,000 cars that enter the zone each day - would be charged.

The mayor hopes a third of these behemoths will go off-road.

Given the huge antipathy to SUVs, or 'Chelsea tractors' as they are known, Mr Livingstone dropped a political masterstroke, doubly so as he exempted less-polluting cars, smaller-engined makes such as a Ford Fiesta, Fiat Panda and Renault Clio - vehicles generally under 120g of CO2 per km. Chelsea tractors and sports car emit more than 225g.

'I have every sympathy for a Scottish hill farmer who needs his four-by-four to get around,' Mr Livingstone said, 'but there is absolutely no justification for cars producing high amounts of pollution being driven in central London.'

The RAC, a motoring group, told London's Evening Standard the mayor's policy was an example of gesture politics: 'Ken's proposals will increase congestion and do very little to cut CO2 in London - the real polluters are the old bangers kept on the road by motorists who cannot afford to change them.'

Surveys show Londoners change their cars less often, keeping beaten-up stationwagons and people carriers for longer because families do much less mileage.

A spokesman for the business group London First told the Daily Mail: 'This is daft. We know this is election year but encouraging gridlock in the centre of London is no vote winner.'

Mr Livingstone hopes it will be. If wrong, he has a fallback position: a GBP4 charge. And by October, he may well be enjoying his third term.