Thousands of jobs could go in the NHS, putting the health service "under real strain", a union warned today.

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 today.

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 was 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 was 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."

However, Health Secretary Andy Burnham promised that frontline jobs would be protected under Labour.

He told the Daily Mirror: "It would not be right to guarantee every job. But Labour will protect frontline services, which we can't do without protecting frontline staff."

Dr Carter said some trusts would say they were keeping jobs when they were really changing the skills mix, meaning qualified people are replaced with unqualified people.

"That worries us - you could see in some areas the dumbing down and the skill mix diluted to the detriment of the patient."

He said many trusts would not replace people who had retired or were leaving.

"What tends to happen is that people leaving are not replaced.

"It can happen by stealth and that's a technique that we think is frequently used."

Other data collected by the RCN in early April from 287 senior nurses - mostly ward sisters - at 180 trusts found understaffing is now a major issue.

Almost all (92%) said patient care was being compromised by short staffing at least several times a month, with nearly a third (30%) saying that it happened on most shifts.

Most hospital wards are operating with an average of 13% fewer staff than they officially need, the poll also found.

The RCN released the figures on the first day of its annual conference in Bournemouth, which is expected to attract 4,000 nurses.

The data comes a little over a week after a poll among GPs suggested cuts to frontline NHS services were already happening.

More than half (55%) of 370 GPs questioned for Pulse magazine said cutbacks to services were occurring in their local area and another 33% said they were planned.

Liberal Democrat health spokesman, Norman Lamb, said: "It's incredibly short-sighted to think the only way to save money in the NHS is to slash and burn services.

"We know from past experience that this will cost more money in the long term."

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."

A Department of Health spokeswoman said: "The 2010 Budget confirmed that funding for frontline NHS services will rise in line with inflation in 2011/12 and 2012/13 and also confirmed that the NHS will deliver annual efficiency savings of £15 to £20bn by 2013/14 in order to continue to increase real terms resources available for patient care.

"In delivering efficiency improvements, service change proposals must demonstrate a strong clinical case for change, promoting better outcomes for patients."