Theresa May will announce a spending for the NHS tomorrow, to be paid for with the "Brexit dividend".
But the plan has already spectacularly unravelled, even though she hasn't officially announced it yet.
The Prime Minister insisted the NHS will receive an additional £20 billion a year in real terms funding by 2024, meaning a weekly increase of £384 million in real terms, and £600 million a week in cash terms compared with now.
She was keen to promote the idea of the cash coming from a so-called "Brexit dividend" - the money Brexiteers claim we'll claw back once we leave the EU.
Institute for Fiscal Studies (IFS) director Paul Johnson says the dividend doesn't exist.
He says the UK faces a steep exit bill, much of the money that would have gone to the EU has already been promised elsewhere, and the Office for Budget Responsibility has calculated public finances will be £15 billion a year worse off due to Brexit.
Shadow chancellor John McDonnell told BBC Radio 5 Live's Pienaar's Politics the announcement was a "publicity stunt".
He said: "Can you imagine if I'd come forward like this? There'd be accusations of magic money trees, this a magic money forest that's come out this morning."
Even senior Tory Dr Sarah Wollaston, who chairs the Commons Health Committee branded the idea "tosh".
She tweeted: "The Brexit dividend tosh was expected but treats the public as fools. Sad to see Govt slide to populist arguments rather than evidence on such an important issue.
"This will make it harder to have a rational debate about the 'who & how' of funding & sharing this fairly."
Referring to controversial promises made by the Leave campaign during the referendum, Mrs May told BBC1's The Andrew Marr Show: "Some people may remember seeing a figure on the side of a bus a while back of £350 million a week in cash.
"Well, I can tell you what I am announcing will mean that in 2023-24, there will be about £600 million a week in cash, more in cash, going into the NHS."
Mrs May said the money, which would not be used for social care, would amount to an increase of 3.4%, and said the country would also have to "contribute a bit more" to health service funding.
Asked where the non-"Brexit dividend" element of the funding would come, Mrs May said: "As a country we will be contributing more, a bit more, but also we will have that sum of money that is available from the European Union."
It is expected that taxes and borrowing will rise to pay for the increase in funding, and resources will be redirected from the more than £9 billion a year the UK currently pays into the EU.
The move comes as Mrs May faces another turbulent week in Parliament on the Brexit front with Tory rebels again threatening to defy her over how much influence MPs will have over any withdrawal deal.
Labour's shadow chancellor John McDonnell said: "After eight years of Tory cuts and privatisation, the NHS and social care are in crisis.
"Today's announcement on funding is nowhere near good enough and the Prime Minister has confirmed there is no new money for social care.
"This falls far short of the 4% that experts say the NHS needs, it is just a standstill, and the Tories are refusing to say where the money will come from.
"You can't trust the Tories with the NHS."
Director of the Institute for Fiscal Studies, Paul Johnson, told BBC1's Sunday Politics: There isnt a Brexit dividend.
"So there isnt a dividend in two senses; first over this period, if you look at the arrangement weve come to with the European Union in terms of paying our exit bill, and you add to that the commitment the Governments already made to keep funding farmers and so on, there is literally arithmetically no money, and in addition we know, because the Governments accepted this, that the public finances will be worse as a result of the Brexit vote.
"The OBR has said by 15 billion a year; it could be a bit more, it could be a bit less.
As a pure, sort of arithmetic point of view, over this period, theres no money.
"If you look further down the road, yes well stop paying money to the European Union, but the economys already shrunk a bit as a result of the vote; the Government has accepted that.
"Actually the public finances will be 15 billion or so worse off, not better off, so there really just isnt money there for a Brexit dividend.