The government is spending £5.5bn less on health in England than it suggested it would be at this stage, the Institute for Fiscal Studies says.
Plans set out in the 2019 election campaign indicated the budget would increase by 3.3% a year above inflation during this Parliament, the IFS said.
But despite extra being put in to cover the high inflation seen, spending had risen by only 2.7% a year on average.
The government defended its record, saying it was making extra investment.
Funding had reached record levels and was making a “real difference” in cutting waiting lists, the Department of Health and Social Care said.
The health budget for this year stands at £179.6bn.
Most of this is going on the NHS but it also includes money for public health, social care and training.
The IFS said England was not unique in facing this challenge, given the high rates of inflation globally, and had, in this Parliament, increased health spending more than many parts of Europe – including Northern Ireland and Scotland but not Wales.
But the pressures on the NHS and the commitment to increase staffing made under the 15-year NHS workforce plan, backed by both the Conservatives and Labour, meant tough decisions would be needed.
More than 40% of public-service spending was already on health, IFS research economist Max Warner said.
And to keep increasing the budget, other areas of government spending would have to be cut.
“Whichever party takes office after the next election, budgets for the next fiscal year and the choice of how much to give to the Department of Health and Social Care will effectively dominate everything else,” he said.
“Neither the Conservative Party nor Labour Party have been keen to set out spending plans.
“But the next government will have to confront this reality – and fast.”
‘Mounting demand’
Saffron Cordery, of NHS Providers, which represents NHS trusts, said the health service desperately needed extra funding.
“The NHS has been through its toughest financial year ever as budgets and services are stretched to the limit in the face of mounting demand and pressure,” she said.
“We can’t go on like this.”
Labour shadow health secretary Wes Streeting said: “Rishi Sunak has broken every promise he’s ever made on the NHS.
“It appears he has given up on turning the health service around.”
