NHS funding will be less than half of what Government figures project, said the Tory head of the Commons Health Committee.
Sarah Wollaston has accused the Government of misleading numbers on increased NHS funding and that per capita spending will actually fall in 2018/19.
The Government spending review gave a "false impression" the NHS was being boosted by £10 billion over the five years of its spending review, when the real figure was £4.5 billion, said Dr Wollaston.
"You can only arrive at the £10 billion by shifting money from public health budgets, and health education and training, and also by changing the date at which you calculate real-terms increases.
"Yes, you can see how they arrived at the figure, but the real figure, we feel, should be quoted at £4.5 billion, which is considerably less," she told BBC Radio 4's Today programme.
Dr Wollaston also warned of the strain an aging population would put on the NHS.
"Certainly for 18/19 we'll be seeing a per capita fall in funding for the NHS at a time when our demographics, the increase in older people, (will see) a 21% increase in the number of people over 65, for example," she said.
In a letter to the Chancellor Philip Hammond, Dr Wollaston said adult social care was reaching a "tipping point".
Dr Wollaston said that unless there was urgent action, the knock-on effects of increased A&E attendances and longer hospital stays could undermine the whole NHS five-year plan.
"Numerous sources testify to the impact of the real-terms cuts to social care, not only to the vulnerable people who rely on care, but also on NHS services," she wrote in an open letter.
A Government spokesman insisted the £10 billion figure was accurate and that it was "wrong to suggest otherwise".
A Government spokesman said: "The Government has backed the NHS's own plan for the future with a £10 billion real-terms increase in its funding a year by 2020/2021, helping to ease the pressures on hospitals, GPs and mental health services.
"We have also allowed local government to increase social care spending by the end of Parliament, with access to up to £3.5 billion of new support."