Whitehall sources confirmed the prime minister and Chancellor Nadhim Zahawi have given the financing go-ahead for the construction of the Sizewell C nuclear reactor in Suffolk.
Private funding will be sought for the project estimated to cost £20-30 billion.
The Government is then set to make a final decision on public investment early next year, with it expected to buy a 20% stake in the plant, costing up to £6 billion.
Business Secretary Kwasi Kwarteng, who is tipped to be Ms Truss’s chancellor if she wins the Tory leadership race, is said to be “massively” on board with the plan.
But her camp did not appear to be united on the issue, with Chief Secretary to the Treasury Simon Clarke raising concerns with Mr Johnson and Mr Zahawi.
In a letter leaked to the Sunday Times, Mr Clarke suggested that acting now would tie the hands of their successors.
Mr Clarke, who is expected to get a senior Cabinet job if Ms Truss wins, said the costs of Sizewell C is “sufficient to materially affect spending and fiscal choices for an incoming government, especially in the context of wider pressures on the public finances”.