The London congestion charge is being reviewed to acknowledge that many combustion engine vehicles are now more environmentally-friendly than when the levy was introduced.
Mayor of London Boris Johnson confirmed that he will look into the exemption of hybrid cars from the tax after Volvo complained that its low-emission vehicles were being penalised for not being electrically powered.
Johnson has said the charge may be based on the volume of carbon a car emits rather than the type of vehicle, to make the system fairer.
Richard Hebditch, campaigns director for the Campaign for Better Transport, noted that the review could open the floodgates for more cars to be exempt from the charge.
If this happened, he said, the tax would be a deterrent to fewer motorists hence failing to reduce congestion as effectively in the capital
He told the Guardian: "The mayor should say no to this proposal [to make low-emission cars exempt].
"The purpose of the congestion charge zone is not to reduce pollution - though that may be a useful by-product of the zone - but to reduce the number of cars coming in to central London."
Officials say that there are now as many as 20 combustion engine cars that emit the same or less carbon than so-called green vehicles, such as the Toyota Prius - drivers of which do not have to pay the levy.
Meanwhile, it was revealed earlier this month that the new Prius is already attracting thousands of orders.
The T3 version of the car emits 89 grams of CO2 per kilometre and is billed as the world's cleanest and greenest family car.
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