Oxford city centre has been labelled one of the most polluted in the country for the third time in less than a year.

In January, a study by the Chartered Society of Physiotherapy revealed 25 nitrogen dioxide hot spots across the UK, with Oxford city centre fifth worst.

This followed last year's Calor survey claiming air quality in a city centre street was the worst out of 30 locations.

Now, the National Union of Railway, Maritime and Transport Workers (RMT) says the first two months of the year show nitrogen dioxide levels in St Aldates are still well above the government target of 40 micrograms per cubic metre of air. It quoted average nitrogen dioxide levels at the roadside in Oxford city centre as 58 micrograms per cubic metre.

But a transport planner for the county council described the latest survey's findings as "misleading and inaccurate".

The RMT survey quotes Government Air Quality Archive figures to show Oxford as eighth in a table of the 28 worst offenders, behind Glasgow and six different London locations. The RMT has published the Department of Environment, Food and Rural Affairs (Defra) figures to kickstart a campaign to renationalise the country's railways.

Bob Crow, general secretary of the RMT, said: "This study shows that in our towns and cities, levels of nitrogen dioxide are too high and are damaging to those with respiratory and other health conditions.

"An expanded Rail and tram network is essential and public ownership of the railways is necessary to achieve this objective."

Niotrogen dioxide, contained in exhaust fumes, can aggravate respiratory problems such as asthma.

By the end of this year, the Government wants levels in each area of the UK not to exceed more than 40 micrograms per cubic metre.

The highest average reading in the RMT survey was London's Marylebone Road with 92 micrograms per cubic metre.

Celia Jones, transport planner for the county council, said the RMT survey findings were flawed because the 28 locations used monitoring equipment at different types of sites.

Some monitors were at kerbside, within one metre of the kerb, some -- including Oxford -- were at roadside, between one and five metres away from the road, and some were urban background monitors, more than five metres away from the road.

Mrs Jones added that it was unfair to place Oxford in such a list of the worst offenders because automatic monitoring roadside data from other major cities was not included.

"If roadside data from Birmingham, for example, had been included, then the readings would be much worse than Oxford's, so these tables present a distorted picture," said Mrs Jones.

"Steve Pryle, the consultant who provided the data for the physiotherapists, has done so again for the RMT, even though we complained that the data was flawed because when he assessed these locations he was not comparing like with like."

Mrs Jones said city and county councillors were preparing an Air Quality Action Plan which would be submitted to the Department for Transport next year, as part of the Local Transport Plan.

An RMT spokesman said: "Defra monitors the quality of our air in a scientific manner and our report on the levels of nitrogen dioxide recorded so far this year points to areas of the country where there are problems that need to be addressed.

"The RMT believes that decisive action to get people out of their cars and into far less polluting public transport will begin to address these problems, and that bringing railways back into the public sector is a necessary part of that process."