Avis said on Monday that CO2 emissions of its 18,500-unit UK rental fleet had fallen by almost 4% to an average of 157g/km per car between 2005 and 2007, and it hopes to see a further fall over the next couple of years.

This figure is below the Society of Motor Manufacturers and Traders (SMMT) average of 167.2 g/km for new vehicles sold in 2006.

Avis said it had achieved this reduction through working with manufacturers to add more lower emission vehicles to its fleet, as well as increasing the number of diesels in its key volume car groups.

Cars on the renta company's fleet from Citroen, Fiat, Honda and Mazda all generate on average between 133 and 149g/km, while it has been sourcing more 'green' cars higher up the size range.

"We have been working hard to bring down emissions and the work is really starting to pay off," said fleet director Paul Hainsworth.

"Ever since we began working with Future Forests, now known as The CarbonNeutral Company, in the late 1990s the environment has been a key priority for Avis and we intend to keep this progress going in the coming years," he added.

Avis recently introduced 20 Toyota Prius vehicles onto its UK fleet and in February Avis Germany added natural gas VW Tourans to its fleet at 19 locations in 15 cities across the country.

Avis Portugal introduced hybrid cars to its fleet in 2005 and currently operates 50 hybrids across the country, reducing its CO2 emissions by over 50 tonnes. Avis Scandinavia also offers customers the chance to rent environmentally friendly vehicles with over 400 ethanol-powered Saab 9-5 BioPower cars, 75 Ford Flexi-fuel and 20 Toyota Prius vehicles on its fleet.

Avis US announced recently that it would introduce 500 Nissan Altima Hybrid vehicles to its fleet, to add to the 1,000 Toyota Priuses launched earlier this year.