Cars promoted as eco-friendly were criticised yesterday for pumping out up to 56 per cent more carbon dioxide than the manufacturers claim.

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, hailed as Britain’s greenest car when it was claimed that it emitted less than 100 grams of CO2 per km (g/km), 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 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”.

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.

Cars with CO2 emission figures below 100g/km qualify for a free band A tax disc. Band B cars emitting up to 120g/km pay only £35 annual vehicle excise duty a year, compared with £400 for band G vehicles that emit more than 225g/km.

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, leased by David Cameron, the Conservative leader. It managed fuel consumption of 26.7 miles per gallon (mpg) in the road test compared with the claimed 35.8 mpg – 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 super-minis that use the same 1.4 litre diesel engine – the Polo BlueMotion and Seat’s Ibiza ECOmotive.

Auto Expressdescribed carbon emissions as “the yardstick by which a car’s ‘greenness’ is measured,” and said that environmental concerns now made a difference in the car market.

Nevertheless, the testers were impressed by the technological innovations introduced to cut CO2 and said five cars tested could be considered “green” despite fuel consumption – and, consequently, emissions – failing to live up to official figures.

The five were Ford’s Focus ECOnetic, the Mini Cooper Clubman D, VW Polo BlueMotion, Seat Ibiza ECOmotive and Toyota Prius.

Official figures for cars are based on laboratory tests conducted by the manufacturers themselves, importers or independent test engineers. They are a selling point in adverts and are listed by the Department for Transport’s Vehicle Certification Agency in its consumer guide to 365 models on sale.

The Society of Motor Manufacturers and Traders insisted that the industry had “made progress in delivering lower carbon cars”. A spokesman admitted that cars may emit more CO2 under real world operating conditions but insisted that all cars had the same “industry standard” tests. Emma Stanley, of Honda, denied that the Civic hybrid claims were a “gimmick”.

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