One in four NHS wards routinely operate at unsafe staffing levels that threaten patient safety, experts have warned.
Researchers at the University of Southampton said that lessons learned from the Mid Staffordshire scandal have been somewhat lost due to a lack of investment in staffing and a chronic shortage of nurses.
The team also pointed to a dilution in the skills mix on NHS wards as healthcare assistants are used to shore up staffing numbers.
A 2016 study from the University of Southampton warned that, for every 25 patients, substituting just one qualified nurse for a lower-qualified member of staff was associated with a 21% increase in the odds of dying.
The latest study said the number of full-time equivalent nurses employed in NHS trusts has increased by 10% since 2013, while there has been a 30% rise in the number of healthcare assistants and support staff.
"The disproportionate increase in support staff numbers has resulted in a slight lowering of skill mix," the study said.
"Registered nurses (RNs) account for 66% of nursing staff in 2017 compared with 69% in in 2013."
Following the Francis Inquiries, which examined the scandal at Mid Staffs where neglect contributed to the death of patients, the National Institute for Health and Care Excellence (Nice) recommended that a level of eight patients per registered nurse should trigger a review of staffing.
But the latest study, which included questioning 91 directors of nursing in NHS trusts, found a quarter of NHS wards regularly work at this unsafe staffing level.
The researchers pointed to a 10% average vacancy rate for registered nurses across the country, with some trusts reporting a 20% rate.
And while nursing numbers have increased since the 2013 Mid Staffs inquiry, the corresponding growth in patient numbers means there has been no improvement in staffing levels.
Professor Jane Ball, lead author of the study, said: "One of the biggest challenges has been the national shortage of registered nurses (RNs).
"The ongoing national shortage of RNs, and failure to increase supply sufficiently, has not been addressed.
"This failure has prevented safe staffing levels from being achieved.
"Nice identified a ratio of eight patients per RN as a level that threatens patient safety.
"But in our survey of directors of nursing, one in four reported wards were routinely running with this high-risk level."
Prof Ball said that a lack of investment in the workforce meant trusts had a "clear vision of safe staffing but without the sufficient means - in terms of registered nurses - to deliver on it".
Patricia Marquis, the Royal College of Nursing's director for England, said: "Mid Staffordshire showed us the dire consequences of nurse shortages and yet those precious lessons have been forgotten so quickly.
"It will trouble patients and the public today to hear the experts warn again of the deadly risks being run and that some parts of the NHS have one in five posts vacant according to this report.
"Now that there are 40,000 unfilled nurse jobs in England, it is time for ministers and the NHS to get a firm grip on the situation before it deteriorates further.
"The legacy of the Francis Report was a once-in-a-generation opportunity to increase nurse staffing levels across all health and care settings but any short-term progress in hospitals has fallen away. Rising patient numbers are outstripping small nurse increases."
She said researchers were right to raise concerns around the increased numbers of support staff.
"These increases must be matched by rises in registered nurses if we're to keep the full and appropriate mix of skills in care settings," she said.
"The Government should commit to a new law for England to provide accountability for staffing levels for safe and effective care, and provide an additional investment of at least £1 billion in nurse education to retain the existing workforce and train the next generation of nurses."
The Southampton study was funded by the National Institute for Health Research.