Owners of the vehicles that cause the most pollution could see a sharp reduction in the cost of driving. Alistair Darling, the Transport Secretary, said that it would be too complicated to vary the charging rate according to engine efficiency.

“If we make it more complicated it would be very difficult to get it going,” he said yesterday. “The object of road pricing is to free up roadspace. The simpler it is, the better the chance of it working.”

He was speaking as he announced plans to introduce a radical system of motoring taxation in which drivers would pay £1.34 per mile on the most congested roads.

Mr Darling confirmed that he was considering abolishing both fuel duty and road tax and introducing a single tax based on distance travelled and the level of congestion. “What we can’t have is one charge on top of another charge. It would be instead of. This is actually a different system of charging.” He said that the idea was to persuade a proportion of drivers to travel at less congested times or switch to public transport.

The cheapest rate would be 2p a mile, which would apply to most roads most of the time. But the tolls would become progressively more expensive according to the level of congestion, with the top rate reserved for peak periods and pinch points on the network. The minister refused to rule out using tolls to raise the overall cost of motoring. Motorists paid more than £42 billion in taxes last year, including £22 billion in fuel duty and £5 billion in road tax.

He was also unable to guarantee that the average motorist would not end up paying more. He said: “That’s a question I can’t answer at the moment. The shape and form of any scheme we introduce is years off. My objective is not to penalise motorists but to get a better distribution of traffic to get more out of the network.”

Environmental groups voiced concern that charging all cars the same rate would remove the existing incentive that fuel duty gives to drive a more fuel efficient vehicle.

A Range Rover V8 does only 12 miles to the gallon in towns and produces 389g of carbon dioxide per kilometre. By contrast, a Toyota Prius, a “hybrid” car that has both a petrol engine and an electric motor, does 56mpg and emits only 104g/km. A Nissan Micra diesel does 50mpg and produces 119g/km.

Tony Bosworth, a transport campaigner at Friends of the Earth, said: “It would be absurd for gas guzzlers like the Land Rover Discovery to pay the same rate as a Mini because people would have no incentive to drive a more efficient car.”

“Road-pricing has a role to play in reducing traffic and cutting emissions, but it must run alongside other taxes and policies to reduce traffic and cut transport’s contribution to climate change.”

Mr Darling said that the Government would look at “other measures to encourage the take-up of environmentally friendly cars”. He said that these could include incentives to manufacturers to produce more-efficient vehicles.

He invited local authorities to bid for extra public money for transport schemes in return for helping to test the new charging system. He said that a trial area would be chosen in the next two years, with a pilot up and running in five to six years. The system would be introduced nationwide in ten to fifteen years.

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