Europe's top court has ordered the UK to act more quickly to reduce levels of air pollution.
In ClientEarth v The Secretary of State for the Environment, Food and Rural Affairs, the group ClientEarth asked the courts to require the UK government to show how it planned to comply with the European Air Quality Directive, which set limits for the levels of pollutants in the air.
The UK missed a 2010 deadline to reduce emissions of nitrogen dioxide, requesting a postponement to 2015 for just over half of its polluted zones.
However in the most polluted areas, including Greater London, the UK did not request an extension, indicating that it would comply with the limits between 2015 and 2025.
Following ClientEarth's application, the UK Supreme Court asked the Court of Justice of the European Union (CJEU) to rule on whether a member state was obliged to apply for the deadline to be postponed.
In its judgment today the CJEU said that a state may apply for a five-year extension - but only if it had drawn up an 'air quality plan that demonstrates how the limit values with be met before the new deadline'.
Alan Andrews, a lawyer at ClientEarth, said the ruling 'provides a clear indication of what the law means by "as soon as possible". This victory is an important step for the fight against air pollution in the UK, and we are also seeking partners across Europe who want to bring cases to protect their right the breathe clear air.'
ClientEarth's case will now return to the UK Supreme Court for a final ruling next year. ClientEarth expects the court to order the Department for Environment, Food & Rural Affairs (Defra) to take action to meet limits in a much shorter timeframe.
At the time of the 2010 deadline, nitrogen dioxide limits were exceeded in 40 of the 43 zones set up in the UK for the purposes of the air quality directive. Diesel fumes are the main source of this gas, which is linked with heart attacks and asthma.
The CJEU's landmark ruling is the first on the directive.