The NHS faces a period of job cuts, clogged hospital wards and rising waiting times, nurse leaders have said.
The Royal College of Nursing said patients should be concerned about the future - despite the health service escaping the worst of the cuts.
The union said it had identified almost 27,000 posts it believes are under threat in the UK and warned that services could suffer in the future.
But NHS chiefs said any savings being made should not harm patient care.
The NHS in England was one of only two areas of spending not to be cut in the recent spending review.
Instead, it was given the equivalent of 0.1% annual rises for the next four years once inflation is taken into account.
But the RCN said because of things like the ageing population and the rising price of drugs the costs for the NHS were rising at a much higher rate than that.
In recent months, the union has been asking for evidence of job cuts from its members as well as asking freedom of information requests and analysing board papers.
In July it reported 10,000 posts could go, but said this figure has now increased to 26,841, the majority of which are front-line clinical posts.
But the union said the true figure could be much higher as it was clear some trusts were not being transparent about their plans.
Peter Carter, general secretary of the RCN, said: "Right now, staff are not only concerned about losing their jobs, they are concerned about keeping services open and how they will cope if they stay.
"The public should be really concerned about the potential impact to the NHS. I predict waiting will rise. It won't be too long before people start asking what is going on."
He said the cuts to local government budgets could lead to hospitals not being able to discharge patients because social services will be too stretched.
He also questioned whether it was right to be trying to make savings at a time when the NHS in England was undergoing one of its most ambitious reorganisations - primary care trusts are being scrapped and GPs given control over local budgets.
He said this could be a "recipe for de-stablilisation".
NHS chief executive Sir David Nicholson said staffing was a matter for local trusts.
But he added: "We have made it clear that efficiency savings must not impact adversely on patient care, and that every penny saved must be reinvested in support of front line services and improving quality."
The RCN also highlighted examples of waste that had been reported. These included issues such as TVs being programmed to come on when no-one is around to watch them and central heating malfunctions which mean they stay on all the time.
It even cited a rehabilitation clinic which had new wet rooms and height-adjustable baths installed at a cost of more than £60,000. Within months, services were moved and the facilities are no longer being used.
Shadow health secretary John Healey said: "This RCN report is an early warning of the strains that the NHS is under and looming problems for staff and patients."
BBC News website readers have been sending their reaction to this story. Here is a selection of their comments:
As a retired NHS nurse, my observations are that there has always been waste in the NHS. It is over-managed and over-specialised, with specialist nurses in every field but never actually of real value. As for the Royal College of Nursing they are all talk and no action. Cathy, Nantwich
Nursing professionals are probably in a better position to assess the impact of the funding freeze than any politician. This is another Con-Dem attempt to divert complaints and public anger away from central government, who implement policies at a local level where they have to adhere to it. Andrew, Birkenhead
Job cuts and NHS budgets are a real concern. I'm a third-year student nurse due to qualify at the end of this year and already having difficulty getting a job. One job I applied for, the post was withdrawn due to the funding for that job being cut. Adam, UK
With the cap on the amount trusts can earn through private patients being removed it means that as waiting times rise, those with money will be able to jump to the front of the queue as hospitals prioritise private patients over NHS patients. Once again, the poorer members of society are going to suffer the most. Alan McDermott, Worthing
If it is true that we are to lose vital front-line services, then this government should be put in the dock for defrauding the British public once again. As for Nick Clegg, he should hang his head in shame alongside Cameron for going back on their pledges. Geoff Shannon, Wirral