The European Commission is taking the UK to court for having too high levels of nitrogen dioxide from diesel engines.
Britain was supposed to meet EU limits on nitrogen dioxide in the air by 2010, but the Government doesn't expect the country to comply with the standard until 2025.
The EU says that levels of nitrogen dioxide, mainly from diesel engines of cars and trucks, are "excessive" in many British cities, and it says that this gas can lead to major respiratory illnesses and premature deaths.
Controlling the amount of this gas in air has proved particularly difficult for the UK.
This is the first instance of the EU taking action against a member state for such a breach.