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Motorists wanting to buy an environmentally friendly car should choose a diesel model and forgo energy-draining luxuries such as air conditioning, according to a government campaign that ranks vehicles according to their carbon emissions.

The Department for Transport is making an unprecedented intervention into the new car market today by telling drivers which are the “greenest” models in each class. Four of the five cars chosen by the DfT to launch the campaign are diesel-powered. The only petrol car is the Toyota Prius hybrid, which runs on a battery at low speed but, at £17,800, costs £3,000 more than conventional cars in its class.

The campaign’s underlying message is that a diesel car is the best option to help to save the planet, unless you can afford a hybrid. A diesel car will typically travel at least 20 per cent farther for the same amount of fuel as a similar-sized petrol car.

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The car with the lowest carbon dioxide emissions is the VW Polo Blue-Motion. It emits 99g of CO2 per kilometre, just under the 100g/km cut-off point for the zero road-tax rating. But it achieves its top ranking only because the manufacturers have removed the air conditioning. Volkswagen plans to sell 900 Blue-Motion Polos this year. This compares with 38,000 conventional Polos, which emit at least 124g/km.

The Low Carbon Vehicle Partnership said that air conditioning reduced fuel efficiency by more than 10 per cent in many models. But it said that manufacturers could make bigger savings on emissions by reducing the weight and power of cars.

The DfT has identified the ten lowest-emission cars in each of 14 size categories, ranging from small cars to 4x4s and luxury cars, and lists them on its website: www.dft.gov.uk/ ActOnCO2.

All 30 of the larger family cars, estates and people carriers listed are diesel, as are the top five luxury cars.

Diesel sales have grown rapidly since 2000, from 15 per cent of all new cars sold to 39 per cent last year. But the rise has begun to tail off, partly because the pump price of diesel tends to be a few pence a litre higher than that of petrol.

The Society of Motor Manufacturers and Traders, which will host the DfT’s campaign launch, called on the Government to lower the duty on diesel to reflect the lower emissions of diesel engines.

Chris Macgowan, the SMMT’s chief executive, said: “Giving a duty incentive to switch to diesel is one of the quickest ways for the Government to achieve its CO2 targets.”

In France more than 70 per cent of cars are diesel because, as in most other European countries, the fuel is subject to a much lower rate of duty than petrol.

But the Energy Saving Trust said that diesels still caused more air pollution than petrol cars, despite the introduction of particulate filters. There was also a limit to how much diesel could be extracted from a barrel of crude oil without a big increase in the energy consumed in the refining process.

Nigel Underdown, the trust’s head of transport, said that there would be a greater environmental benefit from doubling the top rate of road tax for the least fuel-efficient models from £300 to £600.

He called on the Government to produce similar CO2 rankings for secondhand cars and proposed that tax discs should be colour-coded to give buyers an instant indication of a vehicle’s relative efficiency.

The Government’s existing policy of encouraging drivers to buy low-emis-sion cars by giving discounts on road tax is questioned in a new report by the transport consultancy Steer Davies Gleave.

The report argues that giving discounts on either road tax or fuel duty could result in more damage to the environment because they make it cheaper to own and use cars. People take advantage of the discounts to buy more cars and drive farther than they otherwise would have done.

The report concludes: “The most effective way of capping or reducing fuel consumption would appear to be to increase the cost of buying and running standard cars, while not reducing the costs of ‘green’ cars.”