The cost of building the Sizewell C nuclear power plant in Suffolk has doubled since the plans were presented to the UK government in 2020 and could now reach close to £40bn, according to reports.

A rise in construction charges over recent years, combined with cost overruns and delays at EDF’s Hinkley Point C nuclear project in Somerset is expected to increase the final bill to build a successor project at Sizewell, according to the Financial Times.

A report cited people close to the talks between EDF and the government, which are focused on how to finance the nuclear project. The Treasury is expected to decide whether to back the project in this year’s spending review.

According to the report, one senior government figure and two well-placed industry sources said that the cost of building Sizewell C would be about £40bn in 2025 prices.

A government spokesperson said: “We do not recognise this speculative figure – discussions with investors are ongoing and commercially sensitive.” EDF has also been contacted for comment.

In a statement, Sizewell C’s co-managing directors, Julia Pyke and Nigel Cann, said the claims about Sizewell C’s costs were “not accurate and do not reflect the significant savings we are already making” following its work on Hinkley Point C.

“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. At times, gas provided more than 60% of our power needs, pushing up electricity prices and adding to our carbon emissions. 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,” the pair said.

France’s top audit body warned on Tuesday that the country was “far from ready” to build the six nuclear reactors announced by the president, Emmanuel Macron, in 2022.

In a new report, the court of auditors said delays were already accumulating at Sizewell C. It recommended that EDF should secure new investors for Hinkley, reducing its financial exposure to that project, before making a final investment decision on Sizewell C.

The fresh concern over costs has emerged weeks after the Labour donor and green energy entrepreneur Dale Vince challenged the government’s new value for money tsar to examine the costs of a nuclear power.

Vince wrote late last year to the chair of the government’s new Office for Value for Money (OVfM), David Goldstone, to say that the nuclear plant had rising costs and would “will saddle consumers with higher bills long before it delivers a single unit of electricity”.

The UK government and the French state-owned company EDF will fund about 40% of the Sizewell C project, which is planned to generate enough electricity to power 6m homes. Government officials are now gauging interest from private investors to meet the rest of the costs.

The government has proposed using a different financial model to support Sizewell than was used to back the Hinkley Point scheme, and is expected to be given the final go ahead in June’s review of public spending.

Under Hinkley’s agreement EDF can only begin to earn revenues from the project when it begins generating power, which has put enormous financial strain on the company due to escalating construction costs. But under the new framework EDF would be able to recoup its costs as soon as it starts construction, which has raised concerns that consumers and taxpayers will be on the hook for delays and overruns.

“If Hinkley Point C is anything to go by, Sizewell C really should have rigorous financial scrutiny,” Vince wrote. “Originally priced at £18bn, the cost of Hinkley has ballooned to £46bn and then there’s the delays. Back in 2007, the then EDF chief executive Vincent de Rivaz said that by Christmas 2017 we would be using electricity generated from atomic power at Hinkley. We’re now in Christmas 2024 and Hinkley isn’t due to be completed until 2031.”

The government spokesperson said: “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.”