CARS promoted as eco-friendly were criticised yesterday for pumping out up to 56pc more carbon dioxide than the manufacturers' claims.
Three models, including the Honda Civic hybrid, performed so badly in tests that their environmental claims were dismissed as a gimmick.
A further five vehicles, including Volkswagen's Polo BlueMotion, failed to match the claims made by their makers.
Road tests were carried out by 'Auto Express' magazine, which accused manufacturers of attempting to cash in on concerns about global warming.
David Johns, the British magazine's editor, said that demand for eco-friendly cars was rising rapidly but it could be hard for consumers to determine what was "truly green or just pure gimmick".
The survey comes as the Government here prepares to introduce an emissions-based car tax system from July.
Almost a quarter of new cars now claim a CO2 rating of less than 140g/km. Those with a figure below 120g/km accounted for one in 20 sales last year -- it is thought that there would have been more, given a better supply.
The Honda Civic hybrid, regarded widely as one of the lowest emitting cars, performed the worst in the tests.
Instead of the 109g/km of CO2 claimed in the makers' specifications, it was found to put out 171g/km. The testers said its electric motor was "not strong enough to propel the oddball four-door Civic on its own" and they concluded that the vehicle "failed to match the firm's economy claims".
The second car labelled a gimmick was the Lexus GS450h. It managed fuel consumption of 26.7 miles per gallon (mpg) in the road test compared with the claimed 35.8mpg -- meaning higher carbon emissions. Diesel rivals were said to "produce similar emissions and better economy".
Skoda's Fabia Greenline was condemned because its emissions were higher than two other less bulky superminis that use the same 1.4 litre diesel engine -- the Polo BlueMotion and Seat's Ibiza ECOmotive.
A motor industry spokesman admitted that cars may emit more CO2 under real world operating conditions but insisted that all cars had the same "industry standard" tests.
Honda denied that the Civic hybrid claims were a "gimmick".