Pembrokeshire could overtake Neath Port Talbot as Wales' top emitter of carbon dioxide if plans for an £800m gas fired power station proceed, it is claimed.
FoE says Neath Port Talbot emits 6.2m tonnes of CO2 annually
Friends of the Earth (FoE) said the plant proposed for Pembroke Dock would double the county's CO2 emissions.
Pembrokeshire emits 4.4m tonnes a year - second in Wales to Neath Port Talbot.
But John Davies, the leader of Pembrokeshire Council, which is keen to attract the plant, accused the environmental group of scaremongering.
RWE npower is examining sites at Pembroke Dock and in Nottinghamshire for the plant and an application for Pembrokeshire is currently with the Department of Trade and Industry (DTI).
It would be powered by natural gas supplied from the nearby LNG terminals at Milford Haven to produce electricity.
FoE said government figures show that Pembrokeshire currently produces around 4.4m tonnes of carbon dioxide a year. Neath Port Talbot produces 6.3m tonnes.
Gordon James of FoE Pembrokeshire said: "The power station would be very large and we've worked out it would emit over 5m tonnes of carbon dioxide a year.
"Gas is a cleaner fuel than oil but because of the crisis in climate change all these fuels have to be burnt in the most efficient way possible.
"The best way to generate electricity using LNG is combined heat and power where we use the heat locally as well as generating electricity.
"It would be a smaller power station but would be much more effective.
"Many people will be surprised Pembrokeshire generates so much carbon dioxide but we all know there is a major fossil fuel industry here."
But Pembrokeshire Council is keen to attract PWE npower to the county as the power station is expected to create at least 100 full-time well paid jobs, plus another 1,000 during construction.
Councillor Davies said: "The claim (by FoE) is beyond belief in my opinion.
"There have been as many as four oil refineries in Pembrokeshire for a lengthy period - over 30 years.
"The beautiful attributes of Pembrokeshire have been undiminished during the 30 years."
RWE npower said the plant would be capable of generating enough electricity to supply around three million homes.
It said the "state-of-the-art" station would produce significantly less carbon dioxide emissions than coal-fired stations widely in use in the UK.