Energy giant EDF has said the cost of building the Hinkley Point C nuclear power station could jump to almost £21 billion, about £3 billion more than it said last year.
In a statement ahead of its annual meeting, the French group estimated the cost of building the twin reactor plant at the Somerset site would be £20.7 billion.
EDF said it would provide up to £13.8 billion, while Chinese utility CGN will bring £6.9 billion of financing to the project. It said these latest set of figures provided "a contingency margin".
However, in October EDF said the project would cost £18 billion, with it providing £12 billion and £6 billion from CGN.
EDF said that the building plan for Hinkley would take around 115 months (nine-and-a-half years) "after the final investment decision until commissioning of the first reactor".
The French firm had said in October that it expected a final investment decision within weeks and first power in 2025, but that decision has now been delayed until September as the company bids to arrange financing.
This now suggests that if the project completes its financing in September, the plant would not be able to pump out power until the first half of 2026.
This is three years behind EDF's estimate in 2013, that the plant would be operational in 2023.
In a later statement, EDF added that the energy giant and CGN had agreed to commit an extra 15% of "additional capacity", or £2.7 billion, to the project to be used in exceptional circumstances.
EDF added: "We don't expect to use the additional 15% because we expect that Hinkley Point C will be on time and on budget at £18 billion."