The EC has said that the UK has two months to act on reducing the emissions from the plant before the case could be referred to the EU’s Court of Justice.
The coal-fired power station in Wales, owned and operated by RWE, emits more than double the allowable amount of emissions at 1,200mg/Nm3 compared to the limit of 500mg/Nm3, which has been in force since 1 January 2008.
The EC launched infraction proceedings in June against the UK government over the way it has applied EU regulations.
The dispute, which centred on the Welsh coal burned at Aberthaw, could force RWE to close the plant by 2016.
The UK had been granted a relaxation of emissions rules to allow Aberthaw to burn locally mined coal, which is more difficult to ignite than other types of coal due to its low proportion of volatile matter.
The EC is now challenging the interpretation of that provision, saying it was meant to apply only to coal with less than 10 per cent volatile matter. Aberthaw’s feedstock has between 6 and 15 per cent volatile content.
In July, RWE Generation confirmed it was investing £12 million in a new, low-emission boiler, which is expected to reduce the station’s emissions of Nitrogen Oxides (NOx) by more than 60 per cent.