The European Commission is taking legal action accusing Britain of exceeding limits on air pollution from traffic, forcing the UK to introduce tougher curbs or face fines over what is known to cause tens of thousands of premature deaths each year.
The case, introduced on Thursday, says Britain breached EU limits for nitrogen dioxide (NO2) emissions, which cause breathing and other health problems, and gives it two months to respond.
The move ratchets up pressure on Britain following a ruling in May last year in the country's Supreme Court, which found it was breaking the EU Air Quality Directive and asked the European Court of Justice for guidance, due later this year.
A spokesman for Britain's environment ministry said the government was investing heavily in measures to improve air quality such as buying more efficient buses and encouraging more people to use bicycles.
The British case found that air quality improvement plans put in place by the government would reach EU standards only by 2020 in most affected areas or in 2025 in London, 15 years after the original deadline.
Air pollution is believed to be Britain's biggest killer after smoking, causing 29,000 early deaths a year, according to the government's own advisers.
While Britain has a particular problem with NO2, 22 EU member states are struggling to comply with the law, according to a Commission website.
Also on Thursday, the Commission - the EU executive - said Belgium was failing to take adequate steps to rein in its output of particulate matter and called on the government to act to curb it more quickly or face legal auction.