A "final warning" for failing to address repeated breaches of legal air pollution limits has been sent to the UK from the European Commission.
London, Birmingham, Leeds, and Glasgow are among 16 areas with dangerous levels of pollution, according to the body.
The UK is one of five countries served with the warning over persistent breaches of nitrogen dioxide levels, which come from sources including factories and vehicles, and diesel engines.
Air pollution is linked to the early deaths of about 40,000 people a year in the UK - and causes problems such as heart and lung diseases and asthma.
If the countries fail to take action on the issue within two months, the Commission may decide to take the matter to the European Court of Justice.
European Commission spokesman Enrico Brivio told a briefing in Brussels: "It is a warning that we send to member states asking to comply, to take measures to fight this poor air quality that provokes respiratory disease and cardiovascular disease.
"It is an important factor on the health of citizens.
"So we ask the UK, as the other four member states, to take appropriate measures to fight this type of air pollution and we hope they comply."
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."