The City of London has been found to be one of the most polluted places in Europe after monitoring equipment recorded dangerous levels of minute particles for the 36th time this year. Under EU rules, Britain is allowed no more than 35 "bad air" days in the whole year, and now faces court cases and unlimited fines by Europe.

The breaching of the EU levels after just six months will embarrass the government, which was sent a final warning only three weeks ago from the European commission to improve air quality. Many other places in central London are close to the limit and can be expected to break the law within weeks.

The government has applied to Europe for a time extension until 2011 to comply with daily particulate pollution from traffic, but is not certain to be granted it because it has been flouting EU air quality laws since 2005 and is perceived by the environment commissioner Janez Potočnik to have done little to address the problem.

"Air pollution is bad for our health. It reduces human life expectancy by more than eight months on average and by more than two years in the most polluted cities and regions," he said.

The London Liberal Democrat MEP Sarah Ludford said: "This latest breach is yet another wake-up call for the mayor of London and the government. Research has shown that airborne pollution in London could be responsible for up to thousands of premature deaths a year: this is an invisible public health emergency."

Poor air quality is now considered one of the biggest public health issues now facing the UK. A recent report by the House of Commons environmental audit committee included evidence that air pollution could be contributing to 50,000 deaths in the UK a year. A study (pdf) commissioned by Boris Johnson, mayor of London, calculated that more than 4,300 deaths are caused by poor air quality in the city every year, costing around £2bn a year.

Simon Birkett, founder of the Campaign for Clean Air in London, said: "With the first of many London sites breaching the legal limit before the end of June, it is blindingly obvious now why the European commission sent the UK a second and final written warning for breaching these same legal standards, every year since 2005 in London, less than a month ago".

Jenny Bates, London campaigner for Friends of the Earth, said: "Boris Johnson must abandon plans that will make the situation worse, such as scrapping the western extension to the congestion charge, pursuing more river crossings for vehicles and supporting a 50% increase in flights from City airport. This means taking strong action himself, rather than relying on uncommitted government measures to do the job."

A spokesperson for Johnson said: "This is one of several central London locations which will receive a targeted package of measures to tackle pollution, for example applying dust suppressants to road surfaces and deploying the cleanest buses into these areas. Other initiatives include proposed age limits for taxis, converting the bus fleet to hybrid and investing record levels into cycling.

"We are also proposing to include the dirtiest lorries and vans in the London low emission zone by early January 2012. The new bus for London will be 40% less polluting than traditional diesel and we are spending millions to support the mainstream use of zero-polluting electric vehicles."

A Defra spokesman said: "The mayor and London boroughs are responsible for local air quality in London. The mayor has published a draft air quality strategy which includes specific measures to reduce PM10 and NO2 pollution.

"We are confident that PM10 limits will be met in London by the 2011 deadline and the government has submitted evidence to the European commission to demonstrate this."