The European Commission is investigating the British government's approval of a 2GW gas-fired power station in Pembrokeshire following complaints by environmental campaigners.
RWE npower's £1bn Pembroke power station opened last month and is considered as the largest of its type in Europe, capable of powering 3.5 million homes in the UK.
Friends of the Earth Cymru filed a complaint with Brussels, which wrote to the Department of Energy and Climate Change (DECC) asking it to clarify the impact of the power station on surrounding coastal water.
The environmental agency says that the power plant's cooling system will waste energy by dumping water that is eight degrees warmer than the surrounding water into the highly protected Milford Haven.
In addition, the group is arguing that the energy wasted by the system is equivalent to 40% of Wales' electricity demand.
Friends of the Earth Cymru director Gareth Clubb was quoted by BusinessGreen as saying that "Time and time again we have pointed out that the UK government acted unlawfully in allowing this technology to be used in Wales, which isn't considered good enough in the US or in England."