Ministers must honour the pledge given by Brexit campaigners to pump an extra £350 million a week into the NHS, pro-EU MPs have demanded.
More than 40 MPs, who have branded themselves "progressive", have written to Chancellor Philip Hammond, calling on him to signal that the money will be available in next month's Autumn Statement.
The letter, organised by the Vote Leave Watch campaign, states: "In your speech to the Conservative Party Conference earlier this month, you said that the message of the referendum result had been 'received, loud and clear' by the Government. Members of the Government talk of the 'mandate' from the voters for Brexit.
"We accept the verdict of the British people. Yet it is clear that, if this mandate is to mean anything, it must include the single most visible promise of the Leave campaign - spending £350 million more a week on the NHS.
"In just under a month, you will present your first Autumn Statement. We are calling on you to commit to increase national NHS spending by £350 million a week - that is £18.2 billion a year - as soon as this money becomes available by leaving the European Union.
"This additional funding must be over and above the amount that is currently planned to be spent on the National Health Service.
"Anything else will be a betrayal of the wishes of the British people. We challenge you, when you stand up in the House of Commons on November 23, to show us the money and commit to Vote Leave's promise; or explain why you cannot, and why your Cabinet colleagues so cynically misled the British people."
Vote Leave Watch chairman Chuka Umunna said the 41 MPs had written to the Chancellor to make sure Cabinet figures like Foreign Secretary Boris Johnson keep their campaign pledge on the NHS.
"The message to Philip Hammond is loud and clear - this Brexit Government will not be able to run away from the promises of Brexit campaigners.
"He has a month to work out how the Government are going to find the money to keep this promise. If he can't, I hope his pro-Leave colleagues will explain why they pulled the wool over the eyes of so many voters during the campaign," the Labour MP said.