Front-line clinical jobs in the NHS are under threat in England, a union says.
A Royal College of Nursing analysis of 21 NHS trusts where cuts were taking place found more than half of posts under threat were in areas such as nursing and midwifery.
RCN leader Peter Carter said cutting thousands of doctors and nurses could have a "catastrophic" effect on care.
But deputy prime minister Nick Clegg maintained radical reorganisation of the NHS is needed.
While the health budget has been protected, savings still have to be made because of the rising demands linked to the ageing population, new drugs and lifestyle factors like obesity.
A target of £20bn has been set by 2014-15 - a saving of about 4% a year - which ministers have insisted can be achieved through cutting management costs and by front-line services becoming more efficient.
But the union, which released the findings at the start of its four-day annual conference in Liverpool, said its research showed this was not happening.
Some of the proposed changes that mark the biggest shake up of the NHS to date have been opposed by one of Mr Clegg's closest advisers.
Lib Dem MP Norman Lamb has said the plans pose a major "financial risk" to the NHS, and has threatened to quit unless ministers rethink some aspects, such as "the rush" to give GPs control of budgets.
Speaking to the BBC's Today programme, Mr Clegg insisted the changes, including some cuts, were necessary, but said the finer details of the plan were up for debate over the next couple of months.
The RCN has been gathering evidence about the number of posts under threat in the NHS for nearly a year.
It believes there are now 40,000 posts which could close in the next three years, a rise from 27,000 at the end of last year. Most of these will not be redundancies as the NHS tends to rely on natural turnover from people retiring or changing jobs.
As part of its latest research, the RCN took an in-depth look at 21 trusts to see what sort of posts were being targeted.
It showed that 54% of about 10,000 job cuts were clinical and, in total, more than a tenth of the nursing workforce could be lost in these areas.
The union also said it had found examples of services being closed, including rehab centres, detox units and talking therapies.
RCN general secretary Peter Carter said the reforms could "well turn out to be the biggest disaster in the history of our public services" if unions and other organisations were not listened to.
"Clinical staff are the lifeblood of the NHS and it is haemorrhaging at an alarming rate. Many trusts are not being transparent by admitting the proportion of clinical jobs being lost.
"From our research we now know the truth - the majority of job losses are front-line clinical jobs, the jobs that matter to patients.
"Cutting thousands of doctors and nurses could have a catastrophic impact on patient safety and care. Our figures expose the myth that front-line services are protected."
But NHS chief executive Sir David Nicholson questioned the findings, pointing out that latest workforce figures up to December 2010 suggested that nurse posts were continuing to rise.
He added: "There is no excuse to cut back on services that patients need when the NHS will receive an extra £11.5bn of funding.
"The NHS does need to become more efficient, but savings must not impact adversely on patient care. We are clear that every penny saved from efficiencies will be reinvested in patient services."