The Sizewell C nuclear power plant in Suffolk is reportedly under review as the Government looks to limit spending.
The new reactor, located some 30 miles north-east of Ipswich, was expected to be built by energy firm EDF.
Boris Johnson promised £700m of taxpayers’ money to the project in his final policy speech in early September as he sought to make energy security part of his legacy as prime minister.
But a Government official has since told the BBC: “We are reviewing every major project – including Sizewell C.”
The total cost of the Sizewell C project could be around £20bn, according to reports.
It is not expected to begin generating electricity until the 2030s; the similar reactor at Hinkley Point C in Somerset began construction in 2016 and will not be online until 2027, although this is partly due to the impact of the pandemic.
Proponents of the site say it can help get the UK to run on zero-carbon power, but others say the cash would be better spent on wind farms or insulation.
The Government is seeking opportunities to reduce spending after the Institute for Fiscal Studies warned last month that the state faces a roughly £60bn financial black-hole following the mini-budget announcement in September.
The Treasury has since pulled back on a number of previous unfunded tax cuts or spending plans.
The Chancellor last month announced that the two-year energy price freeze for all households will now run for just six months.
Jeremy Hunt said at the time that the universal energy price guarantee will finish in April, with the Government launching a review on how to then support bills after this period.