The government has committed £14.2bn of investment to build the new Sizewell C nuclear plant on the Suffolk coastline, ahead of the Spending Review.
Sizewell C will create 10,000 direct jobs, thousands more in firms supplying the plant and generate enough energy to power six million homes, the Treasury said.
Chancellor Rachel Reeves said the "landmark decision" would "kickstart" economic growth, while Energy Secretary Ed Miliband said the investment was necessary to usher in a "golden age of clean energy".
However, Alison Downes, director of pressure group Stop Sizewell C, said ministers had not "come clean" about Sizewell C's cost, because "negotiations with private investors are incomplete".
Once construction work begins, Sizewell C will take at least a decade to complete.
Reeves said it would be the "biggest nuclear building programme in a generation".
Ms Downes added she believed the investment could be spent on other priorities and feared the project would "add to consumer bills".
