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The final price tag for building the planned Sizewell C nuclear power station in Suffolk is likely to reach close to £40bn, according to people close to negotiations on the flagship energy scheme.

That’s double the £20bn estimate given by developer EDF and the UK government for the project by 2020, reflecting rising construction costs as well as the implications of delays and cost overruns at the sister site Hinkley Point C.

The higher estimate is likely to raise questions about the government’s strategy for restoring nuclear power, at a time of growing government finances and cost-of-living concerns.

EDF says that once operational, Sizewell C should be able to supply low-carbon electricity equivalent to around 6 million homes for 60 years.

The Treasury will decide whether to go ahead with the spending review project this year, according to officials.

The UK government and French energy group EDF were the initial backers of Sizewell C but they are trying to raise billions of pounds from new investors, a process that is taking longer than planned.

Earlier this month the Department for Energy Security and Net Zero (Desnz) said it cannot disclose the current cost estimate for the project because it is “commercially sensitive”.

But a senior government figure and two well-placed industry sources say a reasonable estimate for the cost of building Sizewell C would be around £40bn at 2025 prices.

The government has already committed £3.7bn of state funding to the project. Ministers had planned to reach a final decision on the investment by the end of 2024 but were forced to delay it until spring 2025. Now there are speculations in the industry that any deal could be lost beyond the autumn.

Potential investors in Sizewell C include Centrica, Schroders Greencoat, Emirates Nuclear Energy Corporation and Amber Infrastructure Group.

According to accounts published by Companies House last week, Sizewell C “continues to make good progress in negotiations with private investors”.

Alison Downes, executive director of the campaign group Stop Sizewell C, urged the government to “clean up” the “huge real costs” of the project given that households will pay for its construction first through a tax on energy bills. “This secrecy around Sizewell C is unforgivable.”

Dale Vince, a major Labor party donor and founder of green energy company Ecotricity, has written to the government’s new Office for Value for Money warning that the construction of Sizewell “will burden consumers with higher charge before it can produce a single unit of electricity” .

Speaking to the Financial Times, he added: “Nuclear is too expensive, too slow – and too expensive to be put at the end of its life.”

Nuclear power currently supplies about 14 percent of the UK’s electricity and many experts say it is essential to push to cut carbon dioxide emissions to net zero by 2050.

However, all but one of Britain’s current aging fleet of plants is due to close by March 2030, which could be sooner if planned life extensions are not sustained.

Only one new nuclear power station, Hinkley Point C in Somerset, is currently under construction in the UK but it is delayed and over budget.

The project is due to start construction in 2029 at the earliest, and is worth £46bn. That compares with initial expectations from 2016 that it would start by the end of 2025 and cost £18bn.

Sizewell C uses the same European Pressurized Water technology as Hinkley Point C. EDF said Sizewell C should be cheaper in part because lessons are learned and supply chains can be improved.

It is also built using a different financial structure, the regulated asset base model, which is used in the water sector including the financing of new Thames Tideway sewage tunnel. Developers began to be paid during construction from consumer bills, instead of waiting for the plant to be completed.

But there is skepticism within the government about how much less the Sizewell C will cost compared to the Hinkley Point C.

The progress of the project was partly knocked out by the recent resignation on health grounds of Rob Holden as chair of Sizewell C Limited.

A Desnz spokesman said he was not aware of the “speculative” figure of £40bn for Sizewell C’s value.

“The project is expected to reduce the cost of the electricity system, improve our supply of safe home power and generate significant investments throughout the country.”