The government's decision to back a new Sizewell C nuclear power plant failed to assess the environmental impact of the project and should be overturned, campaigners have argued at the High Court.
Protest group Together Against Sizewell C has launched a bid to challenge development consent granted by the then Business Secretary Kwasi Kwarteng last July.
At a hearing in London, lawyers for the Suffolk residents argued that the government failed to assess the impact of providing an "essential" water supply to the project and did not consider "alternative solutions" to meeting its energy and climate change objectives.
They also argue that the government concluded the power station site would be clear of nuclear material by 2140, when rising sea levels and storm surges could flood the site before it has been decontaminated.
The government, supporting the project with a £700m stake, argues that it made "legitimate planning judgments" and that the campaigners' "unarguable" challenge should be dismissed.
French energy giant EDF says Sizewell C is expected to generate low-carbon electricity to supply six million homes.
But campaigners say the approval went against the recommendation of the planning inspectorate and the advice of Natural England.
David Wolfe KC, representing Together Against Sizewell C, said in written arguments that it remained "completely obscure" when a water supply solution could be implemented.
Mr Wolfe said that "uncertainty" over the water supply, coupled with the one of two reactors being operational by the end of 2033 at the earliest, meant that the government had "no sufficient evidence" to concluded the plant could contribute to a target of reducing greenhouse gas emissions by 78% by 2035.
James Strachan, KC, for the government, said in written arguments that campaigners were "simply reflecting disagreements" heard during consultation.
He said there was "nothing unusual" in a project not identifying a utility supply when this is provided by others.
Mr Strachan said the government had considered alternative means of electricity generation adding that the developer would need to show sea defences would continue to protect the site if its life is extended, or provide additional protection.
The hearing is due to last two days, with a ruling expected at a later date.
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