Data from 26 NHS trusts showed thousands of jobs could be at risk as the NHS in England tries to drive down its spending.
The health service was asked to find between 15bn and 20bn in efficiency savings over the next three years to save money across the board.
However, some campaigners say the move is already leading to cuts in front-line services and will cause job losses.
The Royal College of Nursing (RCN), which released the latest figures, said cuts to services and staff were "becoming a reality across the health service as some trusts look to slash budgets".
Findings from just 26 trusts since the announcement on efficiency savings was made show at least 5,600 jobs are at risk.
The head of policy at the RCN, Howard Catton, said the figures that it collected were on the "conservative side".
Some of the figures include forecasts of what might happen, while some trusts have provided a range of potential redundancies.
He added that job losses might be seen in all areas, not just on the front line, such as nursing posts.
While nurses thought it was great that politicians were talking about protecting the front line, the reality for many was that they were going into meetings or receiving letters from chief executives talking about cuts to the workforce, he said.
Such conversations had been going on since Christmas but were "gathering pace this year", Mr Catton added.
Other data collected in early April from 287 senior nurses – mostly ward sisters – at 180 trusts found understaffing was now a major issue.
Almost all (92 per cent) said patient care was being compromised by short staffing at least several times a month, nearly a third (30 per cent) saying that it happened on most shifts.
Most hospital wards are operating with an average of 13 per cent fewer staff than they officially need, the poll also found.
Meanwhile, 76 per cent of those questioned have seen staff vacancies on their wards in the last year and 49 per cent have had a recruitment freeze in place.
Four in 10 (40 per cent) are banned from using bank or agency nurses while 22 per cent said nursing posts on their wards had already been cut since the drive for efficiency savings began.
The RCN released the figures on the first day of its annual conference in Bournemouth, expected to attract 4,000 nurses.
The figures come a little over a week after a poll among GPs suggested cuts to front-line NHS services were already happening.
More than half (55 per cent) of 370 GPs questioned for Pulse magazine said cutbacks to services were occurring in their local area and another 33 per cent said they were planned.
Among services being affected were psychiatry, blood testing and end-of-life care.
Dr Peter Carter, general secretary of the RCN, said: "We are in no doubt that politicians genuinely want to protect frontline services and the vital care they deliver to patients.
"However, despite assurances that the NHS budget will be protected, the reality is that locally, trusts are making deep and dangerous cuts to staff numbers now, with further cuts planned for the future."
Tory Shadow Health Secretary Andrew Lansley said: "We know that by cutting bureaucracy and the tick-box targets, we can free nurses to care.
"Because we will increase the NHS budget, savings will be reinvested in frontline care and improvements.
"The NHS has seen billions wasted; it's time to give the staff the chance to make changes and improve care for patients."