Sue Roaf, emeritus professor of architectural engineering at Heriot Watt University, said it was madness to build a new power station near flood risk sites.
She warned the proposals on the Suffolk coast risked lives and could transform parts of the area into a “toxic wasteland”.
“It’s ridiculous the government is even considering another power station on the coast,” Prof Roaf said.
“You can downplay the future risk, but even by conservative estimates sea levels will have risen by a metre by 2100, potentially making Sizewell a nuclear island during storm surges.”
EDF Energy, which operates Sizewell B and is expected to submit final plans for Sizewell C in the coming months, said its assessments of flood risk already took into account extreme high tides and sea-level rises.
A spokesman said EDF had considered climate change using the “worst case, but plausible sea-level rise” forecasts – and its sea defences could adapt, if needed.