A £1bn power station under construction in Pembrokeshire could be breaching European environmental law, claim protesters.
Friends of the Earth has complained to the European Commission after the UK government gave backing before full environmental assessments.
RWE npower called consultation on the Pembroke plant "thorough and robust".
The Environment Agency says it has finished public consultation and will now "fully assess" the application.
The Department of Energy and Climate Change (DECC) said it had not yet been notified of any complaint after it gave planning permission in February 2009.
Building work started in May 2009 and the 2000 Mw gas-fired power station is due to be completed in 2012, providing around 100 jobs and energy for more than 3m homes.
However, Friends of the Earth Cymru said planning permission was given before the environmental assessments were carried out.
It believes the power station could damage marine life in the Pembrokeshire Marine Special Area of Conservation and is calling for Welsh Environment Minister Jane Davidson to call in the application.
It said it was "greatly concerned" in particular that a water cooling system proposed would harm fish.
Gordon James, director of Friends of the Earth Cymru, said: "Pembrokeshire's greatest asset is its magnificent natural environment and wildlife.
"This is why the Milford Haven Waterway and adjoining coastal areas have been granted one of the highest forms of legal protection as the Pembrokeshire Marine Special Area of Conservation.
"We're greatly concerned that the water cooling system proposed for the new power station will harm fish and other marine species in the haven.
"This technology is not considered the best available in view of the environmental damage it causes. There are better systems used elsewhere, and the Milford Haven Waterway deserves the best possible technology."
Its complaint is being supported by Plaid Cymru MEP Jill Evans, who said she wanted the planning consent for the power station investigated.
"The Cleddau estuary and the Milford Haven waterway are both protected areas and home to an abundance of wildlife," she said. "It's vital that EU laws are enforced to protect these valuable sites.
"That is why I want the planning consent for the new Pembroke power station investigated.
"If it does allow the pumping of warm water into the Cleddau estuary then the EU Habitats Directive has been broken and the UK government must answer for that."
The Environment Agency, which is carrying out the assessment into the environmental impact of the plant, said: "Our public consultation on the company's application for an environmental permit to operate the plant came to an end some weeks ago.
"The next stage is for us to fully assess the application and consider the responses to the consultation."
A Welsh Assembly Government spokesperson said all requests to call in projects were "treated seriously and considered on their merits within the legal and policy context which applies to them".
The spokesperson said: "However, in view of Welsh ministers' legal duties it would not be appropriate to comment further at this stage."
A spokeswoman for RWE npower said the energy minister granted planning consent "after a thorough and robust consultation process".
"The decision was welcomed by the first minister of Wales for the employment opportunities and diversity it would bring to Wales' energy needs," she added.
A DECC spokesperson said: "Until the EU notify the government that a complaint has been made and we know its terms we cannot speculate on how the government will respond."