THERESA May has been accused by one of her own MPs of an “ill thought-through” £21 billion NHS cash boost, rushed out to coincide with its 70th anniversary, as she insisted tax hikes to help pay for it would be fair and balanced.

The UK Government faced more criticism for saying Britain’s “Brexit dividend” would contribute to the extra spending - that will see Scotland get a knock-on £2bn - with Liberal Democrat MP Layla Moran insisting the suggested windfall was “as real as the Loch Ness monster”.

In a Commons statement, Jeremy Hunt was jeered when he mentioned how some of the new NHS investment would come from not sending membership subscriptions to the EU after Brexit.

But the UK Health Secretary insisted the cash boost would also come from a "putting the economy back on its feet dividend" and a "deficit reduction dividend".

Earlier in a keynote speech at a London hospital, the Prime Minister declared how the NHS was now her government’s “number one spending priority”.

"We cannot continue to put a sticking plaster on the NHS budget each year. So we will do more than simply give the NHS a one-off injection of cash,” she explained.

Under the plan, NHS funding south of the border would grow on average by 3.4 per cent in real terms each year from 2019/20 to 2023/24 by when the budget would have increased by £20.5bn compared to today.

“That means it will be £394m a week higher in real terms,” insisted Mrs May.

She pointed out how the cash boost would mean a knock-on windfall of £1.8bn for Scotland and, while the Scottish Government could spend the money as it liked, she urged Edinburgh to use it “to improve the NHS and to develop their own long-term plans for NHS Scotland…”

The PM added: “This way the vision I have set out today can benefit the whole United Kingdom.”

But Mrs May was conspicuously coy about which taxes would be hiked to help pay for the NHS cash boost, saying only taxpayers would have to “contribute a bit more in a fair and balanced way to support the NHS we all use".

The 2017 Conservative manifesto promises to continue to raise the personal tax-free allowance threshold and the higher income tax threshold towards 2020. It also makes clear VAT will not be increased.

While a pledge not to raise National Insurance Contributions was famously included in the Tories’ 2015 manifesto, a similar promise was not contained in the 2017 document.

Whitehall officials are said to have already looked at the 1p rise on NICs to boost health spending introduced in 2002 by the then Labour Chancellor Gordon Brown.

One Tory backbencher argued it was time to “depoliticise NHS funding” with a dedicated tax for health spending. Noting how people were living much longer, he stressed: “The public do not expect or want an NHS on the cheap. The time is right for the hypothecation of tax for the NHS.”

A Conservative colleague was more critical of Mrs May’s announcement, saying it was “ill thought-through and rushed in time for the NHS at 70”.

He added: “Yes, the NHS does need more money but this should have been found within the context of an overarching reform. Trying to out-do Labour on health is never going to work. We should have been the grown-ups and started a genuine wide-ranging debate about the future of the NHS and how we fund it.”

Earlier, Downing St defended the “Brexit dividend” claim, saying it was a "matter of fact" there would be money to spend, which would be saved from Britain paying £9bn a year into EU coffers.

But the respected economic think-tank, the Institute for Fiscal Studies, has pointed out that by paying the £39bn divorce together with other funding pledges, the financial arithmetic meant there would effectively be no Brexit dividend.