Image copyright PA Image caption Campaigners took legal action after the UK breached EU limits on NO2 emissions

The UK's highest court has ruled that the government must take immediate action to cut air pollution.

The ruling is a significant victory for campaigners, who began legal action after the UK breached EU limits for nitrogen dioxide (NO2) in the air.

Diesel vehicles are a key source of so-called NOx emissions, and NO2 is linked to a range of respiratory illnesses.

The Environment Department said work had already been started on revised plans to meet EU targets on NO2.

In a unanimous ruling, a panel of five judges, headed by the court's president Lord Neuberger, ordered "that the Government must prepare and consult on new air quality plans for submission to the European Commission... no later than December 31 2015".

The case had been brought by ClientEarth - a group of environmental lawyers - which notes that air pollution causes tens of thousands of premature deaths a year in the UK.

Announcing the decision, Lord Carnwath said: "The new government, whatever its political complexion, should be left in no doubt as to the need for immediate action to address this issue."

Analysis: Helen Briggs, environment correspondent

The official number of early deaths from air pollution in the UK is put at 29,000 a year - more than obesity and alcohol combined. According to scientific experts, this may be an underestimate as it does not include all air pollutants, including NO2, which is produced by emissions from diesel-powered vehicles.

The UK has been breaching legal limits for nitrogen dioxide since 2010 in 16 different cities and regions. The judgment forces the next government to draw up new air quality plans - for submission to the EU - by the end of the year.

This represents a considerable challenge - under existing plans, nitrogen dioxide limits set by the EU would not be met until 2030. Campaigners say the government should be looking to cities such as Paris, where there is free public transport in towns and cities on days of high air pollution.

It may also have to consider measures such as low emission zones and congestion charging across the UK, working with local authorities and devolution partners.

Image caption A panel of five judges ruled unanimously on the case

ClientEarth lawyer Alan Andrews said: "This ruling will benefit everyone's health but particularly children, older people and those with existing health conditions like asthma and heart and lung conditions.

"The next Government, regardless of the political party or parties which take power, is now legally bound to take urgent action on this public health crisis."

A spokesperson for the Department for Environment, Food and Rural Affairs (Defra) said: "Air quality has improved significantly in recent years and as this judgement recognises, work is already underway on revised plans (since February 2014) to meet EU targets on NO2 as soon as possible.

"It has always been the government's position to submit these plans before the end of this year. Meeting NO2 limits is a common challenge across Europe with 17 member states exceeding limits."

ClientEarth lawyers recently told a hearing that enforcement by the court was the only "effective remedy" for the UK's "ongoing breach" of European Union law.

The group says that under existing plans, some areas such as London, Birmingham and Leeds would not meet pollution limits until 2030. The original deadline for meeting the targets, set in the European Commission's Air Quality Directive, was 1 January 2010.

In May 2013, the Supreme Court declared that the UK had breached its obligations under the Directive.

The case was then heard in the Court of Justice of the European Union (CJEU), which ruled that national courts can force governments to comply with EU rules.

The CJEU empowered the UK courts to take any necessary measure to ensure compliance.

The legal action then returned to the Supreme Court on 16 April this year.

Defra said that average roadside concentrations of NO2 had fallen 15% since 2010. It added that both NO2 emissions and background concentrations had more than halved in the 20 years from 1992 to 2012.

It said the UK was compliant with EU legislation for nearly all air pollutants, but admitted there had been a significant challenge meeting NO2 targets.

The department added that the court's ruling had not supported ClientEarth's assertion that the government breached Article 22 of the Air Quality Directive by failing to apply for an extension on meeting NO2 limits.

Mike Hawes, chief executive of the Society of Motor Manufacturers and Traders (SMMT), said: "Policies to improve air quality should focus on encouraging the uptake of the latest low emission vehicles, while also addressing other sources such as electricity generation and heating, which between them account for more than half of NOx emitted in the UK (Defra figures).

"Today's diesel engines are the cleanest ever: high tech filters capture 99% of particulates and NOx emissions are down 84% since 2000."

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