Four out of 10 NHS finance directors do not believe the service can continue to provide the same quality even with the extra funding promised by the Government.
Ministers have said NHS spending will rise by £8 billion by 2020/21 - although it is unclear as yet how this increase will be phased.
A new poll of finance directors working in NHS trusts and clinical commissioning groups (CCGs), which handle local budgets, found 43% do not think the NHS can continue to deliver the current levels of quality even with the promised rise in funding.
Meanwhile, 56% of directors surveyed by the Healthcare Financial Management Association (HFMA) said quality can only be maintained if the £8 billion funding is frontloaded.
Almost all (94%) said the money is needed no later than in the next 18 months.
The survey of more than 200 finance directors in England also found 84% do not have sufficient money to implement the five-year plan put forward by Simon Stevens, the head of the NHS.
And 88% said they lacked the confidence that their organisation could deliver the 2% to 3% a year productivity gains needed to close the expected £22 billion NHS funding gap.
Paul Briddock, director of policy at the HFMA, said: "Our report confirms the financial problems in the NHS are systemic and across the board, with particular and immense pressure being felt on the acute provider side.
"The scale of deficit reported is unprecedented. The NHS is not living within its means, which has consequences.
"With just days left before (Chancellor George Osborne's) spending review, finance directors are telling us they're not confident in the current plans and need more clarity."
A Department of Health spokesman said: "We are committed to the values of the NHS which is why we have invested £10 billion to fund the NHS's own plan for the future and spending as a proportion of Government spend has increased in every year since 2010.
"While the NHS is busy, the cost-controlling measures we have introduced like clamping down on rip-off staffing agencies and expensive management consultants will help to support finance directors and the NHS to ensure every penny is spent on patient care."