The study's findings suggest that carmakers previously exaggerated the time needed to comply with car CO2 limits. 

During 2008, carmakers lobbied to extend the deadline for average new car CO2 emissions to reach 130g/km. As a result the EU postponed the target year from 2012 until 2015.

However according to the report carmakers recorded a record drop of 5.1% in average CO2 emissions last year. Transport & Environment says that, as an example, Toyota has almost met its target for the year 2015, six years in advance. With targets now being discussed for vans, calls from vehicle manufacturers for their targets and deadlines to be revised should be ignored, according to the group.

Jos Dings, director of Transport & Environment, said: 'Three years ago the car industry said it could not deliver car CO2 targets on time but is now set to achieve them years ahead of schedule. Now the same industry is saying van CO2 limits cannot be met; it is time the credibility of these claims was questioned.'

Transport & Environment has argued against the suggestion that the decline in emissions has been mostly due to the financial crisis and the rise of scrappage schemes, which prompted consumers to buy cheaper, smaller and more efficient cars.

Instead, the group concludes that more than half of the reduction in 2009, or close to a 3% improvement in average efficiency, was achieved through better technology, rather than sales of smaller cars.

The report found that five companies (Toyota, Suzuki, Daimler, Ford and Mazda) achieved more than 3% CO2 reductions though the application of new technology, while three others (Hyundai, Suzuki and Fiat) achieved the same as a result of selling smaller vehicles due to the subsidies.

Mr Dings commented: 'This data shows that last year's big improvement in fuel efficiency was not just a one-off caused by a shift to smaller cars; carmakers are adding fuel-saving technologies. So the trend of reduced CO2 emissions is structural and will therefore continue when the market returns to normal. The CO2 regulation is clearly working.'