Theresa May is under pressure to explain how the government plans to provide the NHS with an extra £384m a week after talk of a "Brexit dividend" to help provide the cash was widely scoffed at.

On Sunday, the prime minister faced a backlash after she linked a planned funding boost with the infamous Vote Leave pledge to spend the money Britain sends to the European Union on the NHS instead – and even included a reference to the infamous £350m battle bus.

Conservative MP Sarah Wollaston, chair of the health and social care select committee and a former GP, was among those to attack the plan – branding the suggestion of a dividend to fund the 3.4% increase as tosh.

Mrs May is now under pressure from Labour to back up her comments ahead of a speech in London, with shadow chancellor John McDonnell suggesting her figures do not add up.

The prime minister has so far refused to be drawn on whether increased taxes and higher borrowing will be needed to allow for the £20bn increase to the health budget in real terms (accounting for inflation) by 2024, but will use the keynote address in the capital to explain why the NHS does need greater funding.

The NHS will be growing significantly faster than the economy as a whole, reflecting the fact that the NHS is this government’s number one spending priority, she will say.

This must be a plan that ensures every penny is well spent.

It must be a plan that tackles waste, reduces bureaucracy and eliminates unacceptable variation, with all these efficiency savings reinvested back into patient care.

Mrs May is expected to recall the emergency services’ response to the Manchester attack and her own treatment for Type 1 diabetes as part of a glowing appraisal of the NHS, but the feasibility of the cash commitment is set to come under further scrutiny.

Paul Johnson, director of the Institute for Fiscal Studies, told Sky News that the prime minister’s proposal was flawed because the Brexit dividend did not exist.

In the short run, arithmetically there isn’t because we’re going to continue giving money to the EU and spending money on farmers and so on through to 2023 – there’s literally no money there, he said.

In the longer run, the government has accepted that the economy will be smaller and and tax revenues will be lower as a result of Brexit.

Earlier this year, Chancellor Phillip Hammond warned that public spending could not increase because Britain’s economy was suffering due to uncertainty over Brexit.

As Sky’s senior political correspondent Beth Rigby reports, he will likely be quietly furious ahead of Mrs May’s speech because it will be down to him to work out where the money will come from.

To increase taxes would be to break a manifesto pledge, while funding cuts elsewhere would further frustrate a public and cabinet who have grown tired of austerity.

:: Health Secretary Jeremy Hunt will appear on Sky News’ Sunrise from about 7.20am to discuss the government’s plan.
(c) Sky News 2018: Theresa May under pressure to explain £20bn NHS funding plan