Air pollution levels in Oxford at the start of this year breached international health rules, it has been revealed.

The city’s nitrogen dioxide levels exceeded the limits set by the European Union and World Health Organisation guidelines, an investigation by The Times found.

Levels of nitrogen dioxide, which is linked to breathing problems and cancer, are supposed to be kept below 40 micrograms per cubic metre of air.

But in Oxford the average level of nitrogen dioxide recorded by a Government-run monitoring station between January and the end of March was 48 micrograms per cubic metre.

Diesel cars are among the leading producers of nitrogen dioxide, prompting demands for a crackdown.

The city is one of 26 areas out of 146 that had nitrogen dioxide levels that breached EU legislation and World Health Organisation guidelines.

This is the highest number in breach of guidelines in the past decade, according to figures from the Department for Environment, Food & Rural Affairs (DEFRA).

The Oxford Mail revealed in November that air pollution had gone up in some parts of the city.

The measurements showed the amount of harmful nitrogen dioxide in the most polluted part of the city - St Clements - rose three per cent between 2014 and 2015, from 65 micrograms per cubic metre to 67.

Levels were also on the rise in other parts of the city centre including George Street, High Street and Magdalen Street.

But overall in Oxford roadside levels of nitrogen dioxide have dropped by an average of 35 per cent across the city in the last 10 years.

It comes as the Government is preparing to publish its plans to improve air quality. The High Court ruled last year that the current plan was inadequate — ordering a replacement to be produced by April 24.

A spokeswoman for Defra said: “Improving air quality is a priority for this government and we are determined to cut harmful emissions. Our plans have always followed the best available evidence — we have been clear that we are ready to update them if necessary.”