The UK Government's approval of Pembrokeshire's new gas-fired power station is to be investigated by the European Commission.
Pembroke Power Station opened last month. Owners RWE npower say its generating capacity - rated at around 2,000mw - is enough to power three and a half million homes.
The environmental group Friends of Earth has voiced concerns at the way waste heat from the power station's cooling system will be released into the Milford Haven waterway, citing a major impact on wildlife.
Its complaint - made in July 2010 - was taken up by the EC, which will determine whether European Union rules are being breached.
A spokesperson for the Department of Energy and Climate Change (DECC) told ITV Wales that proceedings "are at an early stage".
RWE npower declined to comment, saying the matter is between the European Commission and the UK Government, adding they obtained consent for the power station after a "very exhaustive process".