The European Commission has launched legal proceedings against the UK for failing to deal with air pollution.
The EU says that levels of nitrogen dioxide, mainly from diesel engines, are "excessive" in many British cities.
The Commission says that this gas can lead to major respiratory illnesses and premature deaths.
Britain was supposed to meet EU limits by 2010, but the government admits that London won't achieve this standard until 2025.
The UK's problem with dirty air stems from the EU's air pollution directive, which came into force in 2008.
It set limits on the the levels of air-borne contaminants, including particulate matter and nitrogen oxides, gases that are produced from the burning of fossil fuels.
They are an important element of ground-level ozone, which can damage human health as well as plants and animals.
Nitrogen dioxide (NO2), which in the main is produced by diesel cars and trucks, can inflame the lining of the lungs and lead to respiratory disease.
It is of particular concern to people living near roadways in big cities and those suffering with asthma.
Controlling the amount of this gas in air has proved particularly difficult for the UK.
For the purposes of air pollution, the UK is divided into 43 zones.
In 2010, when the EU restrictions were meant to come into effect, the levels of nitrogen dioxide were exceeding the limits in 40 of these 43 areas.
Member states were able get an extra five years' grace if they put in place plans to cut levels of NO2. The UK admitted that the limits relating to 16 zones, including London, could not be met by the revised deadline of 2015.
For many of these areas, including Greater Manchester, the West Midlands, Merseyside and Glasgow, the government believes the levels can be reached by 2020.
In London though, they admit it is likely to be 2025.
For the EU, this is far too long. They've decided to launch the first case against a member state for breaching the limits on NO2.
Several other EU members, including France, Sweden, Denmark and Greece have also exceeded the levels, but the EU denied that the UK was being picked on.
"Our priority is to protect public health and the environment," said European Commission spokesman Joe Hennon.
"We think that's what the people of the UK would want as well."
What might have tipped the EU's hand was a ruling by the UK Supreme Court last year.
In a case brought by environmental campaigners ClientEarth, the judges agreed that the government was in breach of an obligation to reduce air pollution.
In the judgement, Justice Lord Carnworth wrote that "the way is now open to immediate enforcement at national or European level."
The campaign group believes that, in addition to the Supreme Court verdict, the scale and the duration of the UK's breaches made the EU action inevitable.
"The UK has some of the worst NO2 levels in Europe, they're a national disgrace," said Alan Andrews, a lawyer with ClientEarth.
"London has a particular problem, in some streets it is three or four times above the legal limits."
The legal process could ultimately end in the European Court of Justice where the UK would face huge fines if found in breach of the directive.
If the government is to cut levels it will need to take drastic actions, say campaigners. Around half of new car sales are diesel powered, they say. There will need to be strict low emissions zones in cities.
"Germany implemented low emissions zones very early," said Alan Andrews.
"They have 60, we just have the one in London and ours doesn't include cars - it's a low standard."
Another option is cutting speed limits.
"The evidence from Germany suggests they can reduce NO2 by 10-15% on heavily polluted roads, but the scale here in the UK is so big they need to be looking at everything possible to tackle the problem," said Mr Andrews.
The UK has two months to respond to the European Commission.
The Department for Environment, Food and Rural Affairs (Defra) said that Britain wasn't alone in breaching the NO2 limits, pointing out that in 2012, 21 member states reported their emissions did not comply with the annual mean target.
"Air quality has improved significantly in recent decades. Just like for other member states, meeting the NO2 limit values alongside busy roads has been a challenge," said a spokesman.
"That is why we are investing heavily in transport measures to improve air quality around busy roads and we are working with the commission to ensure this happens as soon as possible."
A spokeswoman for London Mayor Boris Johnson said: "Since the mayor was elected, the number of people living in areas exceeding nitrogen dioxide limits has halved but he fully recognises the need to take further action.
"This includes the introduction of the world's first ultra-low emission zone in central London from 2020, tougher requirements for taxis from 2018 and a £20m fund to tackle local problem areas."
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