The UK faces being taken to court over air pollution after the European Commission sent a "final warning" for repeated breaches of legal limits in parts of the country.
The warning was issued for persistent breaches of levels of nitrogen dioxide, which comes from sources including traffic, particularly diesel engines, in 16 areas including London, Birmingham, Leeds, and Glasgow.
If the UK and four other countries which received final warnings fail to take action within two months, the commission said it may take the matter to the European Court of Justice.
Air pollution is linked to the early deaths of about 40,000 people a year in the UK, causes problems such as heart and lung diseases and asthma, and affects children's development.
Legal levels of nitrogen dioxide, which allow hourly limits to be breached only 18 times a year, should have been met by 2010.
But the Government has admitted it could be 2020 before air quality levels are within the rules for some areas and 2025 for London, where annual limits were broken on one road just five days into 2017.
Road traffic is responsible much of the emissions, with around four fifths of nitrogen oxides from transport coming from diesel-powered vehicles.
Health workers and environmentalists have ramped up calls for the Government to take action on air pollution, demanding a new Clean Air Act to tackle the problem and a crackdown on diesel vehicles.
Jenny Bates, Friends Of The Earth air pollution campaigner, said: "Air pollution is responsible for tens of thousands of early deaths every year and is harming the health of an entire generation of children.
"Current government plans have been shown to be too little too late," she said, calling for Government funding for clean air zones in pollution hot spots to tackle transport emissions.
London mayor Sadiq Khan demanded ministers bring in a national diesel scrappage scheme to get the dirtiest vehicles off the road and give cities more power and resources to tackle pollution.
Environmental law firm ClientEarth has won cases in the Supreme Court and High Court which require the Government to produce stronger measures to bring down illegal levels of pollution as quickly as possible, with new plans due by April 24.
Chief executive James Thornton said the warning was "a reminder of just how serious a problem the UK has with air pollution.
"With this final warning from the Commission, the Government is under pressure from all sides to stop dithering and act decisively to meet its moral and legal obligations to clean up our unhealthy air."
But a Government spokesman said ministers were "firmly committed" to improving UK air quality.
It had committed more than £2 billion since 2011 to increase uptake of ultra-low emissions vehicles and support greener transport schemes, with a further £290 million in November to support electric and clean cars, buses and taxis.
"We will update our air quality plans in the spring to further improve the nation's air quality," the spokesman said.
Labour's Mary Creagh, chairwoman of the Parliamentary Environmental Audit Committee, warned a new Environmental Protection Act would be needed to ensure air pollution standards were not watered down with Brexit.
Commission spokesman Alexander Winterstein warned while the country was still a member of the bloc: "European law applies fully."