EU presses UK over London air pollution
The UK government has just weeks to convince EU officials that it will meet European clean air standards in London, if it is to avoid a court case.
The European Commission is assessing UK data for London and will decide in a month's time whether to give the UK an extension until mid-June to comply.
Last June the Commission gave the UK a "final warning" over air pollution in Greater London and Gibraltar.
Tiny airborne particles, called PM10, are above EU limits in those two areas.
The UK is at stage two of a six-stage infringement process, EU Environment Commissioner Janez Potocnik told reporters in London on Thursday.
He said EU infringement procedures had been opened against 20 of the EU's 27 member states over air quality.
Two countries - Slovenia and Sweden - have been referred to the European Court of Justice over their PM10 levels.
Big fines can be levied if countries are found to have breached EU law.
The Commission says PM10, emitted mainly by industry, traffic and domestic heating, can cause asthma, cardiovascular problems, lung cancer and premature death.
June 2011 is the final deadline before the Commission escalates its legal action against the UK, Mr Potocnik said.
The infringement actions follow the entry into force of a new EU Air Quality Directive in June 2008.
Mr Potocnik said it was not the Commission's job to instruct member states on the methods used to implement the directive.
"States can choose whatever measures they want - they have their own menus. Our concern is the level that should be met," he said.
"We work in good faith with governments. We can't look at all the monitoring stations, check every day," he said.
Last year a scientific study commissioned by London Mayor Boris Johnson estimated that air pollution was responsible for 4,267 early deaths in the UK capital, through long-term exposure.
It found that central London had the worst air pollution in the capital, but outer London had the most early deaths linked to the pollution, due to its higher population.