Oxford is home to the greenest buses in Britain, according to a report released yesterday.

Oxford Bus Company was rated the country's most environmentally-friendly bus fleet, based on the level of engine emissions, out of 114 operators surveyed, for the second year running.

Stagecoach in Oxfordshire, listed as Thames Transit, was ranked ninth, the highest-rated subsidiary of its parent group.

Oxford Bus Company's average combined emission level - including carbon monoxide and nitrogen oxides - was 5.85 grams per kilowatt, compared with Stagecoach's 9.64 g/kW.

The worst-ranked operator surveyed, privately-owned South Gloucestershire Bus and Coach, based near Bristol, had an average combined emission level of 15.79 g/kW.

The national average was 11.91g/kW.

Oxford Bus Company managing director Philip Kirk said: "The new information shows just how serious we are about clean air and how far ahead of the game we are. "

The report, commissioned by the company's parent group, Go Ahead, was carried out by independent transport consultants TAS Partnership.

But Oxford is still failing to meet European air quality targets and the city council is drawing up proposals to ban all but the cleanest vehicles from the city centre by creating a Low Emission Zone.

Mr Kirk said: "Utopian solutions like electrical buses have been tried but are unreliable and can't carry the number of passengers required.

"Alternatives like hydrogen fuel cell vehicles are many years away.

"That's why we're committed to harnessing the best available low-emission technology now, rather than sitting back and waiting for new technology to arrive.

"The technology to reduce nitrogen oxide emissions has only become available in the past year and the Oxford Bus Company was the first operator in the country to buy both buses and coaches with Euro V standard engines that significantly reduce nitrogen oxide."

Chris Childs, marketing manager at Stagecoach, said: "We're committed to playing our part to improve air quality in Oxford, and our investment in new buses contributes significantly to this."

Tony Payne, the city council's environmental protection team manager, said: "As vehicle fleets improve, it becomes more difficult to make further improvements and we're aware of that.

"We're also aware that Oxford Bus Company is leading the field, if you like."

Mr Payne said a Low Emission Zone would have an impact, by targeting the older vehicles in the fleets.

He added: "We're not meeting EU air quality levels and therefore we need to identify where the emissions are coming from."

The leader of the council's Green Party group, Craig Simmons, welcomed the report, but said air quality still needed to be addressed.

He said: "The fact there are lots of buses gives us a cumulative effect that contributes to bad air quality."