Warning over Aberthaw Power Station emissions
The UK could be referred to the European Court of Justice over claims a coal fired power station in the Vale of Glamorgan is breaking emission regulations.
The European Commission says the Aberthaw station has not complied with nitrogen oxide gases limits since 2008.
The UK government has two months to respond to the commission's concerns.
Plant operator RWE npower says new equipment will be installed at Aberthaw from next year.
Nitrogen oxides released into the environment from burning fuels have serious consequences for human health and the environment, causing respiratory illnesses, acidifying soil and surface water, and damaging vegetation.
The commission said the power station near Barry, which has been operating since 1971, currently emits more than double the legal limit of gases.
Tougher regulations on power plants will be introduced in January 2016 throughout the European Union.
'Crazy to close'
Aberthaw employs just under 600 full-time and contract workers and RWE claims it can supply power for around three million homes.
It is estimated to be worth £75m to the Welsh economy each year and it is feared any threat to its running could impact on other Welsh businesses.
Tyrone O'Sullivan, chairman of Tower Colliery in the Cynon Valley, said a challenge to Aberthaw would be a "major threat to Tower and all the Welsh coal producers".
Tower's opencast mine supplies about 650,000 tonnes of coal to Aberthaw, which is three-quarters of its output, according to Mr O'Sullivan.
He said: "Aberthaw is a crucial part of the distribution of electricity in this country.
"It would be crazy to close Aberthaw on this issue. Just look at the amount of coal that is being burned around the rest of the world in countries like China."
RWE npower said power stations across Europe were finding market conditions "increasingly difficult".
"Britain needs long-term cross-party support for both the objectives and the delivery of this country's energy policy to create the kind of market stability needed to make very large, long-term investment decisions," it added.
BBC Wales environment correspondent Iolo ap Dafydd explained: "At the heart of this disagreement may be a different interpretation of the amount of nitrogen oxides which can be emitted from the Aberthaw power plant.
"RWE has already dealt with sulphur dioxide emissions and aims in the next four years to deal with the nitrogen emissions.
"The question is will they be able to comply with the EU regulations soon enough?"