Emissions from new cars have fallen by almost 5% thanks to the popularity of small cars that were bought through the scrappage scheme.
Average CO2 emissions fell 4.7% to 145.2g/km during the first half of 2010, compared with the same period in 2009, according to figures from the Society of Motor Manufacturers and Traders (SMMT)
The SMMT's chief executive, Paul Everitt, said: 'Continued vehicle manufacturer efforts to improve fuel efficiency and cut emissions have further reduced average CO2 output during a better-than-anticipated first half of 2010.
'The remainder of the year is set to be challenging, especially now the scrappage scheme has ended.'
Sales of alternatively powered vehicles reached 11,468 during the first half of 2010, an increase of 50% over the same period in 2009.
Registrations of eco-branded cars, such as the VW Bluemotion or Ecoflex from Vauxhall, have also soared to 74,000 vehicles so far in 2010, 141.4% up on the first half of 2009 and 850% more than during the same period in 2008.