UK referred to Europe's top court over air pollution
The UK is being taken to court by the European Commission over its long-standing failure to meet EU limits for nitrogen dioxide (NO₂).
Germany, France, Italy, Romania, and Hungary have also been referred to the court for breaching pollution levels.
The European Environment Commissioner, Karmenu Vella, said the EU "owed it to its citizens," to take legal action.
The UK has promised a comprehensive air pollution package shortly.
But Britain could face fines totalling millions of pounds, on-going until the problem is solved
The government has already lost a series of battles in the UK courts on air pollution.
Commissioner Vella said it was the Commission's responsibility to ensure people could breathe clean air.
He said the member states being taken to court had been repeatedly warned to clean up pollution as soon as possible.
"We can't possibly wait any longer. It's high time to intensify efforts and end exceedances (of pollution levels)."
Environmentalists say by taking the UK to the European Court of Justice, the EU has demonstrated what will be lost after Brexit.
The Environment Secretary, Michael Gove, previously promised that governance of the environment would not be diluted when the UK leaves the EU.
But he revealed last week that the UK environmental watchdog he proposes after Brexit would not have the automatic power to take the government to court.
The Green MEP Keith Taylor welcomed the EC's decision. He said: "The Commission is being forced to take legal action against the UK because the government remains steadfastly apathetic in the face of a public health crisis that is linked to the deaths of 50,000 British citizens every year."
"Post-Brexit, this is exactly the kind of scrutiny and oversight the Tories plan to escape. Proposals for a so-called environment watchdog that is nothing but a lame lapdog without the legal teeth to take the government to court put this reality in sharp relief.
"This is particularly concerning as legal action by the Commission and environmental lawyers, on the basis of EU law, has been the only way to force the UK government to take any action on air pollution at all."
Margherita Tolotto from the green group European Environment Bureau said: "European air quality laws are being broken on a continental scale.
"Everyone in Europe has the same right to clean air, and when national governments fail to deliver EU protections, it's right that the European Commission steps in to protect us from the air we breathe.
"Today's announcement should surprise no-one. The countries being sent to court have had too many final warnings."
As the announcement was being made, lawyers for Paris, Madrid and Brussels were in front of the European Court of Justice asking that the three cities be allowed to challenge vehicle emissions regulations set by the European Commissions and agreed by national governments.
They are trying to annul the Commission regulation that allows diesel vehicles to exceed emissions limits during road tests, in the wake of the "dieselgate" scandal.
A Defra spokesperson said: "We continue to meet EU air quality limits for all pollutants apart from nitrogen dioxide, and data shows we are improving thanks to our efforts to bring levels of NO₂ down.
"We will shortly build on our £3.5bn plan to tackle roadside emissions with a comprehensive Clean Air Strategy, setting out a wide range of actions to reduce pollution from all sources."
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