The government, the largest shareholder in the project, said Sizewell C, in eastern England, will cost around £38 billion ($51 billion) to construct.
The project will also be funded by Canadian pension fund La Caisse, British Gas owner Centrica, Amber Infrastructure and French energy giant EDF.
“It is time to do big things and build big projects in this country again,” Energy Secretary Ed Miliband said in a statement.
“Today we announce an investment that will provide clean, homegrown power to millions of homes for generations to come,” he added.
The plant, which has been in financial limbo for over a decade, is not expected to start generating electricity until the 2030s. The projected construction cost of £38 billion exceeds previous official estimates of £20 to £30 billion — and campaigners have warned that further cost overruns or delays could impact households.