The UK government is being taken to court over claims pollution levels emitted by a power station in the Vale of Glamorgan are above legal limits.
The European Commission says coal-fired Aberthaw, near Barry, has been pumping out more than double the legal amount of nitrogen oxides for seven years.
Nitrogen oxides can cause problems for people with respiratory illness.
A UK government spokesman said: "Air quality has improved significantly - we have invested £2bn since 2011."
He added: "Aberthaw Power Station is also investing to meet future emission limits set by the EU.
"It would be inappropriate to comment on this case further while infraction proceedings are under way."
The Aberthaw plant is operated by RWE npower and regulated by the UK government.