Campaigners opposed to the building of a new nuclear power plant near Sizewell in Suffolk are embroiled in the latest stage of a legal battle with the Government.
Protest group Together Against Sizewell C objects to a decision, made in 2022 by then business secretary Kwasi Kwarteng, to give the development the green light.
The group lost a High Court fight in June and have now asked three Court of Appeal judges to consider their concerns.
Sir Keith Lindblom, Lady Justice Andrews and Lord Justice Lewis are overseeing a Court of Appeal hearing in London, which is due to end on Thursday.
Barrister David Wolfe KC, who is leading Together Against Sizewell C's legal team, told appeal judges on Wednesday that the central issue relates to whether a "development consent order" was lawful "without any assessment" of the environmental impacts of an "essential" fresh water supply.
A spokesman for Together Against Sizewell C said in a statement outside court: "It is clear that the business secretary needed to guarantee how a permanent water supply of two million litres per day for Sizewell C would be obtained, before giving consent.
"However, the environmental impact of such a plant was not included in the planning application for the nuclear power plant, and therefore was neither assessed nor taken into account."
The group has taken legal action against Energy Security and Net Zero Secretary Claire Coutinho and Sizewell C Ltd.
Lawyers representing the two defendants say the appeal should be dismissed.
Together Against Sizewell C argued at a High Court hearing in March that the Government failed to assess possible environmental impacts, including the impact of the water supply, and did not consider "alternative solutions" to meeting energy and climate change objectives.
The Government argued that it had made "legitimate planning judgments".
A High Court judge dismissed Together Against Sizewell C's challenge.
French energy giant EDF, which is due to develop the plant, has said Sizewell C is expected to generate low-carbon electricity to supply six million homes.
Ministers have said the multibillion-pound project will create 10,000 highly-skilled jobs, with its go-ahead being welcomed by unions and the nuclear industry.