The European Commission has sent a "final warning" to the UK for failing to address repeated breaches of legal air pollution limits.
Air pollution limits were breached in 16 areas including London, Birmingham and Glasgow.
The UK is one of five countries served with the warning over persistent breaches of nitrogen dioxide (NO2) levels, which come from sources including factories and vehicles, particularly diesel engines.
It is reported an estimated 50000 Britons die prematurely every year from illnesses associated with pollutants such as NO2 and ozone.
The UK has been at the forefront of resistance to laws aimed at curbing air pollution deaths, and in violation of the EU’s NO2 limit since 2010. Parts of London overshot the annual air pollution limit for the whole of 2017 in the first five days of the year.
A referral to the European court is not inevitable and penalties may depend on mitigating factors, such as steps being taken to align with EU law.
But if the UK does not satisfy the European Commission that it is acting to reduce its emissions, it could face a lump sum fine, a penalty payment, daily five-figure fines, or a combination of these.
