The UK Government's project to develop a new £18bn nuclear power station has been branded "risky and expensive" by the public spending watchdog.
The National Audit Office (NAO) said the economic case for Hinkley Point C was "marginal and subject to significant uncertainty" at the time the deal was finalised by ministers last September.
The report said consumers are being "locked-in" to subsidising the Somerset development through their energy bills until 2030, despite the fact the cost of electricity will likely be cheaper by then.
Payments set to be added to consumer bills have soared from an estimated £6bn to £30bn, the report said.
The Department for Business dismissed the report's claims, however, insisting that building the station was an "important strategic decision".
Amyas Morse, head of the NAO, said: "The department has committed electricity consumers and taxpayers to a high cost and risky deal in a changing energy marketplace.
"Time will tell whether the deal represents value for money, but we cannot say the department has maximised the chances that it will be."
A spokesman at the Department for Business, Energy and Industrial Strategy said: "Consumers won't pay a penny until Hinkley is built; it will provide clean, reliable electricity powering six million homes and creating more than 26,000 jobs and apprenticeships in the process."
A spokesman for EDF Energy, the French state-owned company building the plant, said the report showed Hinkley Point C remained good value compared with alternative choices.
"Relaunching the UK nuclear new build industry at Hinkley Point C will enable costs for future projects, in particular Sizewell C, to be lower," he said.