The European Union's top court ruled on Thursday that Britain had breached the bloc's air pollution limits for years and ordered it to comply with the rules, raising the possibility that the country may be fined even though it has quit the EU.
Britain left the EU, including the orbit of the European Court of Justice (ECJ), at the end of last year. However, it agreed to apply ECJ judgments in any cases initiated while it was still an EU member.
In its first ruling against Britain since Brexit took place, the court ruled that the country had "systematically and persistently" breached EU limits for nitrogen dioxide (NO2) in 16 areas, including London, Manchester, the urban area of Teesside in northeast England and Glasgow from 2010 to 2017.
London also breached the hourly limits for NO2, and Britain failed to take measures to keep breaches of pollution limits as short as possible, the court said.
It ordered Britain to reduce NO2 pollution to below the EU limit. Failure to do so could see the European Commission take further legal action with the imposition of financial penalties.
Britain's environment ministry said it was considering the ECJ ruling.
"Air pollution at a national level has reduced significantly since 2010, and now we are out of the EU we are continuing to deliver our 3.8 billion-pound ($5.3 billion) air quality plan," a spokesperson told Reuters.
Road transport is the main source of NO2 emissions, meaning urban areas tend to be hit hardest.
Prolonged exposure to air pollution can cause damage to the eyes, diabetes, lung disease and cancer and even death. An inquest last year found that air pollution contributed to the death in 2013 of a London schoolgirl, 9-year-old Ella Kissi-Debrah, who suffered from severe asthma.
In a bid to curb pollution and meet climate goals, Britain has pledged to ban the sale of new petrol and diesel cars and vans from 2030.
London will also expand its ultra-low-emission zone this year, requiring vehicles to meet strict standards or face daily fees.
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