The planned Sizewell C nuclear power station in Suffolk could surge in cost to nearly £40 billion, according to reports.
People close to talks over the energy scheme have estimated the rise in costs, the Financial Times newspaper reported, which is double the forecast by developer EDF just five years ago.
The Government said it did not recognise the figure, while Sizewell C’s managing directors said it is “not accurate”.
Separately, the French state auditor recommended that EDF should not make a final investment decision in the project until it has cut exposure to Hinkley Point C, which it also owns.
Nuclear plants are seen as increasingly important electricity sources as the Government tries to decarbonise Britain’s grid by 2030, replacing fossil fuels with green power.
However, the last time Britain completed one was in 1987, which was the Sizewell B plant.
Hinkley Point C, in Somerset, is under construction and is expected to produce enough power for about six million homes when it opens, but that may not be until 2031.
Sizewell C, meanwhile, is yet to get sign-off.
The Treasury is thought to be mulling whether to give it the green light in an upcoming spending review.
EDF, the French energy giant which owns and runs Britain’s nuclear fleet, and the Government were the first backers of the project.
But they are in the process of trying to raise billions more from prospective investors including Centrica, Schroders Greencoat, Emirates Nuclear Energy Corporation and Amber Infrastructure Group.
One senior Government source and two industry sources reportedly said a reasonable assumption for the cost of building Sizewell C would be £40 billion in 2025 prices.
Engineering teams in December 2023 working on Hinkley Point C’s first reactor building, at the nuclear power station construction site in Bridgwater, Somerset (Ben Birchall/PA)
The increase reflects surging construction cost inflation and the knock-on effects of delays at Hinkley Point C.
A spokesperson for the Department for Energy Security and Net Zero said: “We do not recognise this speculative figure – discussions with investors are ongoing and commercially sensitive.
“New nuclear power stations such as Sizewell C will play an important role in helping the UK achieve energy security and net zero, while securing thousands of good, skilled jobs and supporting our energy independence beyond 2030.
“The project is expected to reduce the cost of the electricity system, boost our supply of secure homegrown power and generate major investment nationwide.”
Joint managing directors of Sizewell C, Nigel Cann and Julia Pyke, said in a statement that the claims are “not accurate and do not reflect the significant savings we are already making because we are building on the achievements at Hinkley Point C”.
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They added: “The real question to ask is what is the cost of not doing Sizewell C?
“This winter we have seen prolonged periods of dull, calm weather and low output from wind and solar.”
They said: “A low-carbon future needs new British nuclear, for lower electricity costs, energy security and thousands of great jobs which will help to transform communities across the country.”
It comes as French national auditor Cour des Comptes recommended that EDF delay its final investment decision in the Sizewell C plant.
The auditor wrote in a report: “La Cour therefore recommends that EDF should not take a final investment decision on Sizewell C before achieving a significant reduction in its financial exposure to Hinkley Point C.”
Any delay to the investment decision by EDF could see costs rise even further.
EDF was approached for comment.