French energy supplier EDF has estimated that the cost of completing the new Hinkley Point nuclear plant will be £1.5bn more than expected.
The company, which is the project’s main backer, said the total cost of the plant was now likely to be £19.6bn.
Hinkley Point C would be the UK’s first new nuclear plant for decades, but has been beset with budget problems.
A review of the project found there was also a risk it could be delayed by up to 15 months.
However, EDF said it still hoped to finish the first phase by the end of 2025 as planned.
The extra costs result from a better understanding of the design, supplier contracts and the volume of work, the company said.
France’s state-controlled energy firm is funding two-thirds of the project, which is expected to create more than 25,000 jobs, with China investing the rest.
Construction is underway after Prime Minister Theresa May approved the project in September last year.
However, critics of the deal have warned of escalating costs and the implications of allowing nuclear power plants to be built in the UK by foreign governments.
Last month the National Audit Office called it “a risky and expensive project”.