The Government's "cash for bangers" car scrappage scheme encouraged buyers to look for more energy-efficient new cars, according to a latest report.
The report from the Society of Motor Manufacturers and Traders (SMMT) said the scrappage scheme was behind the fall in CO2 emission by 5.4% to 149.5g per one kilometre in 2009 compared with the previous year.
The SMMT added that this was the best annual growth since it began records in 1997.
Paul Everitt, chief executive of SMMT, said the scrappage scheme played a vital role in achieving the lower emission figure in 2009.
On an average, the new cars purchased under the scheme emitted 133.3g/km CO2, 26.8% lesser than the average emission of 182.3g/km by the scrapped vehicle.
Mr Everitt said: "Building consumer awareness and delivering effective mechanisms to influence buying behaviour through a long-term environmental tax regime and the Government's recently announced ultra-low carbon incentive scheme, will become increasingly important."
The Mini sector showed the best annual improvement as average emissions in the sector fell by 6.7 per cent to 115.6g/km in 2009, while Luxury car models were the worst pollutants, despite CO2 in this sector dropping by 6% at 250.3g/km.
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