The dispute over the quality of air in Oxford has taken a new turn with the Government wading in to dismiss damning reports as scientifically unsound.

Respiratory physiotherapists had labelled Oxford as one of the UK's worst cities for air pollution. The investigation, by the Chartered Society of Physiotherapy, found that of the 25 nitrogen dioxide hotspots studied Oxford city centre was the fifth worst.

Earlier, Calor Gas claimed that air quality in the city centre was the worst out of 30 locations, and compared spending a day in the city centre to smoking 61 cigarettes. And a recent a study by the National Union of Railway, Maritime and Transport Workers (RMT) found nitrogen levels in St Aldate's were well above the Government target of 40 micrograms per cubic metre of air.

Oxfordshire County Council rejected all three reports, which were compiled by freelance journalist Steve Pryle, who is currently working for the RMT. The council said the reports painted an incorrect picture, mixing data at roadside and background locations.

Now County Hall says the Department for Environment Food and Rural Affairs (Defra) has backed it by branding the surveys "simplistic".

The Defra statement said: "While measurements at each site are made on a comparable basis and are factually correct, it would not be scientifically sound to make simplistic conclusions based on them to derive a ranking as to which towns are the most polluted in the UK.

"This is for a number of reasons, such as the fact that background stations generally represent overall exposure more closely than do roadside sites."

David Robertson, the council's executive member for roads and transport, said: "Spurious surveys that paint a distorted picture will no longer cut any ice.

"We say again, there is no significant health problem in Oxford as a result of air quality. Pollution levels throughout the city are generally described by Defra as low.

But Mr Pryle said: "Defra confirm that the data published on nitrogen dioxide was factually accurate.

"It also confirms that there are potential health effects at roadside sites."