UK government measures to tackle air pollution still don't comply with European Union legislation to improve air quality and meet nitrogen dioxide limits, Britain's High Court ruled on Wednesday.

Nitrogen oxides reduce air quality and member states have been flouting EU limits on a range of pollutants associated with respiratory and other illnesses and more than 400,000 premature deaths per year, according to European Commission data.

Britain's highest court, the Supreme Court, ordered the government last year to come up with a plan to bring air pollution within legal limits as soon as possible.

Environmental law firm ClientEarth said those plans did not go far enough to tackle nitrogen dioxide emissions and launched legal proceedings against the government in the High Court.

In Wednesday's ruling, the court said the government's calculation of future vehicle emissions was ‘too optimistic’ and its plan was inconsistent with measures to improve pollution as soon as possible.

The ruling comes a week after Britain gave the green light to build a new $22 billion runway at London's Heathrow Airport, Europe's busiest airport, which has faced opposition due to worries over noise and air pollution.

Under the EU's Air Quality Directive, member states were supposed to comply with nitrogen dioxide (NO2) limits in 2010, or 2015 if they delivered plans to deal with high levels of the gas, which is produced mainly by diesel engines.

UK government data last year showed only five out of a total 43 pollution zones in Britain would comply by the end of 2015, 15 zones by 2020, 38 by 2025 and 40 by 2030.

The remaining three zones - Greater London, the West Midlands and West Yorkshire urban areas - would not even comply by 2030, the data showed.

Under plans submitted to the European Commission last year, ‘Clean Air Zones’ would be introduced in areas of Birmingham, Leeds, Nottingham, Derby and Southampton where pollution is most serious, by 2020.

Vehicles such as old buses, taxis, coaches and lorries have to pay a charge to enter these zones but private passenger cars would not be charged.

The judge on Wednesday said the British government's planned compliance dates for some cities were chosen to coincide with when it expected to face Commission fines, not with taking action as soon as possible.

A spokesman for the Department for Environment, Food and Rural Affairs said the government was considering the ruling and its next steps.

‘Our plans have always followed the best available evidence. We have always been clear that we are ready to update them if necessary and have been at the forefront of action in Europe to secure more accurate, real-world emissions testing for diesel cars,’ he said.