Thousands of jobs could go in the NHS, putting the health service "under real strain", a union has warned.
Data from 26 NHS trusts shows at least 5,600 posts could be at risk as the NHS in England tries to drive down its spending.
That figure could rise to more than 36,000 in a "worst case scenario" and if the data is extrapolated across all hospital trusts, Howard Catton, head of policy at the Royal College of Nursing said.
The loss of posts - including redundancies and staff not being replaced if they leave or retire - could happen over the next three years, he said.
Job losses may be seen in all areas of the NHS, not just on the frontline, such as nursing posts.
The NHS has been asked to find between £15 and £20 billion in efficiency savings over the coming three years as part of a bid to save money across the board.
Dr Catton said a number of trusts "were deliberately holding back in being clear" on where efficiency savings would strike.
Dr Peter Carter, general secretary of the Royal College of Nursing (RCN), said the drive to find up to £20 billion could lead to longer waits for patients and "less staff being asked to do more".
Dr Carter said he had not seen an analysis of how the £20 billion figure had been arrived at but politicians could not pretend that sort of cash could be removed without an impact on the frontline. He said it was "disingenuous" to suggest you could remove £20 billion from the £100 billion NHS budget and "nothing changes".
He added: "I really can't see that you can take £20 billion out and expect everything to carry on as is it is."