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The long-awaited National Infrastructure Strategy is to be further delayed, and not released next week as expected, the BBC understands.

The detailed 30-year plan was to be published "alongside" the Budget, the government said at the Queen's Speech in December.

Three weeks ago, then chancellor Sajid Javid confirmed the timetable.

The strategy is seen as crucial to the government's plan to "level up" regional disparities.

The delay will allow the new chancellor, Rishi Sunak, to refocus the strategy, to reflect potentially larger resources available, and to incorporate the challenge of achieving "net zero" carbon emissions over the same 30-year timescale.

Treasury sources say the overall ambition to make investments to "level up" the regions that also help meet commitments on climate change, remains and will be reflected in next week's Budget.

The strategy, which foresees spending of £100bn over this parliament, will contain vital funding projections for transport, local growth and digital infrastructure.

After the recent High Court ruling over Heathrow, which found expansion plans had failed to adequately account for policies on climate change, some experts say the government needs to look again at the impact of environmental policy within the provision of infrastructure. There has also been a debate about whether housing should be part of the plan.

The strategy is also the government's formal response to a now two-year-old National Infrastructure Assessment, which was the product of an impartial commission set up when David Cameron was prime minister. It should have been published last autumn.

Publication of the National Infrastructure Strategy should now be expected before May, sources have suggested to the BBC.

The Budget is still expected to include some green lights for high profile infrastructure projects, but the main move in this area will be to set the overall big numbers on capital spending. It is the infrastructure strategy and the Comprehensive Spending Review later this year that will determine the detailed policy.

Shadow chancellor John McDonnell said the delay to the strategy suggested there was "absolute chaos" in the government.

With the threat of climate change and "an economy at risk of recession" the UK needed large scale infrastructure spending to start immediately, he said.