Prime Minister Sir Keir Starmer will give his backing to the chancellor’s Budget in a speech on Monday, and commit the government to going “further and faster” on pro-growth measures.
He will say Chancellor Rachel Reeves’s statement will help to alleviate cost of living pressures, lower inflation and ensure economic stability.
It comes as the Treasury faces questions over whether it was transparent about the state of the public finances in the run-up to the Budget.
The Conservatives claimed Reeves misled the public by being too pessimistic about the economic outlook when official forecasts painted a more upbeat picture.
No 10 has denied that Reeves misled voters and defended her statement.
Despite the Office for Budget Responsibility (OBR) downgrading growth from next year, the prime minister will argue that “economic growth is beating forecasts”, but the government must do more to encourage it.
Protecting investment and public services will further drive financial growth, Sir Keir is expected to say.
The prime minister will also promise to cut “unnecessary red tape” in infrastructure after a report found the UK had become the most expensive place in the world to build nuclear power infrastructure.
He will call for reform in the sector and an urgent correction to “fundamentally misguided environmental regulation”.
Business Secretary Peter Kyle is to be tasked with applying the lessons of the nuclear power report to infrastructure more widely.
The prime minister’s speech on Monday, just five days after the Budget, may suggest some nervousness over how the government’s economic plans have been received by the public, though No 10 say a statement was already planned.
In the days since the Budget, Downing Street has been forced to publicly back Reeves after she was accused by political opponents of repeatedly warning about a downgrade to the UK’s economic productivity forecasts, paving the way for tax hikes.
In a letter to MPs sent on Friday, the chairman of the OBR revealed that he told the chancellor on 17 September that the public finances were in better shape than widely thought.
The Conservatives have accused Reeves of giving an overly pessimistic impression of the public finances as a “smokescreen” to raise taxes.
Tory leader Kemi Badenoch said the letter showed Reeves had “lied to the public” and should be sacked.
Last week, a spokesperson for the Treasury said: “We are not going to get into the OBR’s processes or speculate on how that relates to the internal decision‑making in the build‑up to a Budget, but the chancellor made her choices to cut the cost of living, cut hospital waiting lists and double headroom to cut the cost of our debt.”
Both the chancellor and Badenoch are scheduled to appear on the BBC’s Sunday with Laura Kuenssberg programme.