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Theresa May had earlier denied claims by charity the Red Cross that a shortage of funding has forced the NHS into a "humanitarian crisis."
However, the chief executive of NHS England, Simon Stevens, told MPs that squeezed budgets were directly affecting care.
"Over the next three years funding is going to be highly constrained, he told the Public Accounts Committee.
"In 2018-19, real terms NHS spending per person in England is going to go down, ten years after Lehman Brothers and austerity began.
"We all understand why that is but let's not pretend that that's not placing huge pressure on the service."
He denied previous claims by May that the NHS had been given more funding than it had asked for.
"It is a matter of fact that like probably every part of public services, we got less than we asked for [in the spending review] so it would be stretching it to say the NHS got more."
Stevens' comments follow a clash at Prime Minister's Questions between May and Labour leader Jeremy Corbyn over the crisis currently facing the NHS.
May used the session to attack the Red Cross after they warned that the NHS faces a "humanitarian crisis" this winter. She described the comments by chief executive of the British Red Cross, Mike Adamson, as "irresponsible and overblown" and mocked Corbyn for raising the issue.