Nurses' leaders say patients are being harmed by job cuts in the profession caused by continuing NHS deficits.
Nurses are gathering for their annual conference
The Royal College of Nursing, whose conference opened on Sunday, claims that 22,300 NHS posts have been lost in England in the past 18 months.
Specialist nurses are said to have been particularly hard hit.
But Prime Minister Tony Blair said the union's figures were "greatly exaggerated", and that the number of jobs lost was very small.
In its report Our NHS - Today and Tomorrow, the union said the health service was facing a debt crisis that was "real and entrenched".
The RCN study, compiled from reports by members and NHS board papers, said trusts had been forced to shed 22,300 posts through a combination of redundancies, recruitment freezes and post closures.
The financial crisis was also hitting patient care, the study claimed.
According to the latest government figures, the NHS will have a small surplus overall for the financial year just finished.
Forecasts from the third quarter of 2006-7 showed that the NHS was in line to finish the financial year with a £13m surplus.
This is despite one in three hospitals and primary care trusts predicting deficits.
The Department of Health said the figures being used by the RCN related to the "natural turnover of staff" experienced in any organisation - including posts not being refilled after staff leave, and agency staff not being replaced.
It said the actual number of compulsory redundancies was 1,446 - of which only 303 were clinical positions, such as doctors or nurses.
Mr Blair told BBC One's The Politics Show: "The fact is, today we have a workforce which is 300,000 more than it was in 1997."
As a result, he said, by the end of 2008 the issue of waiting lists would be "effectively dealt with".
RCN general secretary Dr Peter Carter said he stood by the RCN's figures.
"The deficits issue is not history - it is real, entrenched and continues to hit patient care, services and jobs.
"Yes, the NHS achieved overall financial balance last year - but at what cost?"
He added: "This is hitting services, hurting patients, undermining staff morale and threatening the hard-won progress made over recent years."
Howard Catton, head of policy at the RCN, told BBC Five Live that the union's figures had been calculated by monitoring the number of posts "which have been deleted, vacancies that haven't been filled and some redundancies".
The RCN claims specialist nurses, which have been trained to provide expert care in areas such as diabetes and heart disease and have a range of enhanced powers like prescribing, had been particularly effected.
A poll of 807 specialist nurses for the report found one in five were facing a risk of redundancy, while half were aware of cuts in their specialist area.
June James, who has been working as a specialist diabetes nurse for the last 12 years, said: "Posts are being downgraded and services cut. I think it shows a lack of respect for the job we do."
Michael Summers, from the Patients Association, told BBC Five Live that callers to his organisation's helpline said there were "not enough nurses to go round" and patients were "in fear of infections".
"What is clear is that those who leave are not being replaced," he said.
Liberal Democrat health spokesman Norman Lamb described the RCN report as "devastating" for the NHS.
"Nurses are the backbone of the health service - it is terrible that they are suffering due to this government's failures," he said.
And shadow health secretary Andrew Lansley said it confirmed the Conservative Party's fears that "financial mismanagement would lead to job cuts and damage to services for patients".
The RCN, which represents 400,000 nurses, published its report to kick start its annual conference in Harrogate.