Hospitals are running wards with a dangerously low number of nurses and are using healthcare workers as "stand-ins", an investigation suggests.
Analysis of two years' worth of official data shows almost every acute hospital in England is failing to meet its own standard on how many nurses it should have on wards.
Nurses described "desperately unsafe" numbers of registered nurses, while others said patients are being put at risk.
The problem appears to have got worse since the Government introduced a cap on how much hospitals are allowed to spend on agency staff.
Health Secretary Jeremy Hunt ordered NHS trusts to publish monthly data on staffing levels as part of the Government response to the mid-Staffordshire scandal, in which hundreds of patients died as a result of poor care.
But analysis by the Health Service Journal (HSJ), covering data from 2014/15 to 2016/17, shows 96% of acute hospitals (214) reporting figures failed to meet their own planned level for registered nurses working during the day in October 2016.
Some 85% (190 hospitals) also missed their target for nurses working at night in the same month.
This is the worst performance for both day and night since the HSJ started analysing data in 2014.
Across the two years studied, more than 150 hospitals failed to achieve their planned daytime staffing levels for nurses, while the data suggests gaps in rotas are being plugged with healthcare assistants.
Research published in November found that patients are a fifth more likely to die in hospitals where nurses are replaced with less-qualified staff.
Professor Peter Griffiths, from Southampton University, an expert on nursing numbers, said the use of healthcare assistants to fill gaps in the long-term could mean trusts "delude" themselves that they are maintaining safety.
Professor Alison Leary, head of workforce modelling at London South Bank University, told the HSJ: "The overall trend shows organisations bolstering nurse gaps with healthcare assistants.
"This might seem like the only practical solution at the time and could be seen as someone being better than no-one.
"However, when that becomes the norm, we have to consider the inherent risk given the evidence that registered nurses have direct benefits for patient safety."
As part of the investigation, nurses told the HSJ of widespread problems on wards.
One, working in the south west of England, said: "The most patients I have ever had is 16 on a night shift, I know for a fact that I did not do all the observations and can only hope that I did not cause anyone any real harm."
Another incident report from 2015 showed one nurse was left to care for 24 patients on a medical ward.
Another nurse said: "Corners are cut and safety is compromised daily, we just do a really good job of covering it up."
Another said: "Skill mix was desperately unsafe for a normal shift. I felt sick to my stomach."
The HSJ analysis showed that Princess Alexandra Hospital in Harlow has not achieved above 80% of planned nurse staffing since quarter four of 2014/15.
West Cumberland Hospital, part of the North Cumbria University Hospital Trust, and Princess Anne Hospital, run by University Hospital Southampton Trust, had less than 90% of planned nurses working at night across the two years.
Shadow health secretary Jonathan Ashworth said: "These new figures show that almost every hospital in England is now short of nursing staff. Tired, overworked nurses cannot be expected to continue providing the quality of care which patients need."
Janet Davies, chief executive of the Royal College of Nursing (RCN), said patients were being put at "serious risk" by low staffing levels.
She added: "While healthcare assistants play a crucial role in the team, research shows that support roles cannot replace registered nurses if patients are to receive safe care."
The National Institute for Health and Care Excellence (Nice) conducted work into safe staffing levels on wards with a view to setting minimum safe numbers.
But this work was stopped by Mr Hunt, who has given the task to NHS Improvement.
Ruth May, executive director of nursing at NHS Improvement, said: "I've visited numerous hospitals since the New Year and I've seen staff of all disciplines giving everything they've got to provide good care, but there is more we can and should do to help them.
"Right now we need to do more to help get patients back into their homes, supported by social care where necessary.
"We also need to do more to support trusts to recruit and retain more staff."