The Environmental Ratings for Vehicles, created by Cardiff University and automotive consultancy Clifford Thames, looks beyond tailpipe emissions.

As well as CO2, the scale looks at hydrocarbons and particulates emitted from a vehicle.

Weight is also taken into consideration, as well as emissions produced during manufacturing and servicing.

The ongoing goal of the study is to create an independent, unbiased index for motorists to use when considering which vehicle to purchase.

Researchers hope the ERV scale will become for the environment what NCAP standards are for safety.

Cardiff and Clifford Thames' research has also revealed that conventional petrol and diesel cars are beginning to surpass Toyota's Prius on environmental friendliness. While the Prius scores highly on tailpipe emissions, it looses points due to its heavy weight.

So far, it is only lighter conventional vehicles that outpace the Prius, but David Riemenschneider, chief executive of Clifford Thames, believes that vehicles within the Prius's class will be catching up within the next 12 to 18 months.

However, he said the Prius has played an important role putting pressure on other carmakers to further environmental developments.

“Hybrids have really served their purpose well,” said Riemenschneider. “They've brought an increased focus within the industry on improving emissions.”

Riemenschneider does not believe, however, that hybrid technology leading to further development of electric vehicles is necessarily the future for greener cars.

The energy used to produce electricity in the UK accounts for 30 per cent of Britain's greenhouse emissions, while cars produce only 20 per cent, he claimed.

“You would be taking a significant step backwards with electric cars,” said Riemenschneider.