The list advises motorists to consult colour-coded A-G labels in new-car showrooms. These look similar to energy labels used on household appliances and show fuel economy and CO2 emissions.

Car buyers should question whether their prospective purchase has CO2 emissions below the 170g/km average.

Which? suggests hybrid cars and alternative fuels as a serious option to reduce harmful exhaust fumes, but has concerns over availability and costs of both green cars and green fuels.

Car-buyers are advised to check for fuel availability before buying an alternative fuel car. Some fuels, such as bio-ethanol, are still in their infancy so finding a filling station may not always be easy.

Cars like the Saab 9-5 BioPower, costing from 21,867 pounds, can run on E85 bio-ethanol or unleaded petrol but fuel availability means the choice is often taken out of the equation.

"One of the main barriers against adopting alcohol-powered cars at present is the lack of places to fill up on E85 fuel," the Which? report said.

Cost is also a factor. In March, 10 Morrison's stores in East Anglia and the West Country started selling E85 for 2p a litre less than petrol. But sector analysts say bio-ethanol needs to be cheaper than this, as fuel economy is around a third lower than when the car is run on petrol.

GREATER INCENTIVES NEEDED

Which? insists the government needs to introduce greater incentives to convince private and company car drivers to switch to bio-fuels, something already done to great effect in Sweden.

Electricity and hydrogen are hotly tipped as the fuels of the future. However, electric cars are limited by a lack of speed and range and are environmentally friendly only when charged with cleanly generated electricity.

The only electric car sold in the UK at the moment is the tiny G-Wiz city car.

Hydrogen remains a tempting green fuel, since the only by-product is water. Several car manufacturers have developed prototype hydrogen cars but none is yet commercially available.

Initially it is thought the price tag will be steep -- hydrogen fuel cells are still regarded as about 10 times too expensive to be viable.

Cheaper road tax and congestion charge exemption might act as sweeteners for those first in line for the next generation of green cars, the magazine suggests.

Which? picks out the Toyota Prius 1.5 VVTi T-Spirit Hybrid as the greenest petrol car available in the UK, emitting less CO2 than even the cleanest diesel supermini.

A hybrid family car that uses both a petrol engine and an electric motor, the Prius cuts CO2 emissions by 44 percent versus a similarly sized 1.8-litre Toyota Avensis auto.