Thousands of NHS jobs in England are being cut despite government promises to protect frontline services, a union says.
The Royal College of Nursing has identified nearly 10,000 posts - double the number from two months ago.
The RCN said this was just the start of what was shaping up to be a "crude" round of cuts which would harm patient care.
But managers said some job losses were necessary.
While the NHS budget is being protected, the health service has been told to save up to £20bn by 2014 to help it cope with increasing pressures from the ageing population, rising price of drugs and lifestyle changes such as obesity.
Ministers have talked about making the system more efficient by encouraging care to be moved away from hospitals and into the community.
But RCN chief executive Peter Carter said the latest findings suggested that the health system was simply falling back on "crude" and "short-sighted" cuts.
"Our figures expose the myth that frontline services will be protected. If this trend for cuts continues the NHS will soon be straining at the seams."
He said his fear was that specialist nurses, who work with people with long-term conditions such as diabetes and asthma, would be targeted along with nurses working on general wards.
"Local NHS organisations appear to be adopting a slash and burn approach to jobs which is shocking and will have a disastrous effect not only on the quality of care provided but on the range of treatments that are available," Mr Carter added.
In April, the union warned at least 5,600 posts had been lost or were earmarked for cuts based on data from 26 trusts in England.
Those figures have now increased to 9,973 posts and involve 100 NHS bodies in England. Job losses are also being seen in other parts of the UK.
Most of the cuts involve recruitment freezes or not replacing staff who retire, although some trusts are known to be making redundancies.
It is believed both nurses and doctors are being targeted along with a range of support staff.
Alongside job cuts, the RCN also highlighted other measures that were being taken including changing skill mixes by using healthcare assistants instead of nurses.
The union said it would continue to monitor the situation and urged members to report problems to them as part of its new Frontline First campaign.
The findings come just a week after a similar warning from the British Medical Association.
Health minister Anne Milton said NHS managers were wrong to be making such cuts.
"I understand the RCN's frustration and concern. Many trusts are living in the past and interpreting efficiency savings as budget and service cuts. This is wrong. It is about doing more for less."
But Nigel Edwards, of the NHS Confederation, which represents managers, said job losses were inevitable although he agreed they should be part of a range of efficiency measures.
"NHS organisations are looking closely at where further savings can be made, and in some cases this may mean a decision not to fill certain posts or reduce headcount."