The nuclear power station being built at Hinkley Point will start operating a year later than planned and will cost an extra £3bn, EDF has said.
The revised operating date for the site in Somerset is now June 2027 and total costs are estimated to be in the range of £25bn to £26bn.
EDF said it would have no cost impact on British consumers or taxpayers.
The French energy giant developing the site largely put the delays down to the impact of the pandemic.
EDF has published the findings of a review into the cost and schedule of the power station, taking into account the continuing impact of the Covid-19 pandemic.
A reduction in workers allowed on site due to Covid safety measures had resulted in the loss of more than half a million days of critical work in 2020 and 2021, it said.
Stuart Crooks, managing director of Hinkley Point C, said they had to "suddenly cut numbers on site from more than 5,000 to around 1,500".
He added this meant "we had no choice but to become less efficient" with the supply chain "also hit hard".
"In April 2020, 180 suppliers were fully shut down, but even as late as February this year, more than 60 suppliers were operating with reduced productivity due to Covid," he said.
Mr Crooks said that in January 2021, EDF estimated a six-month Covid impact, assuming an imminent return to normal conditions, but then a second wave of Covid-19 hit.
"In total, the start date for Unit 1 has gone back 18 months since construction started in 2016.
"In such a complex project, it wouldn't be credible to say we can measure exactly how much of this is due to Covid-19 impact, but it is clearly in excess of 12 months."
Mr Crooks said "other factors have affected the schedule and costs" and the firm is "facing the same issues as other major projects with UK-wide supply and labour shortages and inflation".
A government spokesman confirmed: "While the Covid pandemic has understandably led to delays, the revised Hinkley Point C construction forecast will have no impact on British billpayers or taxpayers, with any increase in costs borne entirely by the developers."
The spokesman continued: "We will continue to work closely with EDF to bring Hinkley Point C to completion, the first new nuclear power station in a generation, which has created and supported over 22,000 jobs across the UK.
"New nuclear power is essential to the UK's energy security, providing clean and affordable homegrown power."