The UK car industry decreased CO2 emissions by just one per cent year on year in 2007, according to Clean Green Cars, a pro-consumer website.
Average CO2 levels are predicted to fall to 140g/km by 2008, the media monitoring website said.
Although the UK government is aiming to cut emissions to 130g/km by 2012, the website said at the present rate of 1.16 per cent, it would be 2021 before the 140g/km target is reached.
The best performing manufacturer was Mini, which decreased its fleet average CO2 levels by over 17 per cent.
In 2008, the website predicted Mini would have the lowest CO2 levels of any manufacturer.
The other top car makers last year were Citroen, Fiat, Peugeot and Daihatsu.
Jay Nagley, publisher of Clean Green Cars, said the figures for 2007 were "depressing" and demonstrated how far the industry still has to go.
"While lots of money is being spent on high-tech research, we need to see tangible results," he said.
"It is significant that the manufacturer who has made the most progress, Mini has done so not with high-tech hybrids, but with a host of incremental improvements like stop-start technology. New environmental prototypes have their place, but we also need practical action now," he added.
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