The European Commission has sent a “final warning” to the UK over breaches of air pollution limits.
It said limits had been repeatedly exceeded in 16 areas including London, Birmingham, Leeds, and Glasgow.
Germany, France, Spain and Italy were also served with warnings over nitrogen dioxide levels.
The commission said if countries did not take action within two months it could take the matter to the European Court of Justice.
Nitrogen dioxide comes from sources including factories and vehicles, particularly diesel engines.
The commission said more than 400,000 people died prematurely in the EU every year as a result of poor air quality and that millions more suffered respiratory and cardiovascular diseases.
Under EU law, when air pollution limits are breached member states have to implement air quality plans to bring the levels back down.
Friends of the Earth said it was “shameful” that the UK had breached the limits and called for new domestic legislation to protect people from pollution once it leaves the European Union.
Asked whether the UK would remain bound by any legal proceedings after Brexit, Commission spokesman Alexander Winterstein said: “For as long as the UK is a member of the European Union, rights and obligations apply.
“European law applies fully.”