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.
Following the Francis Inquiries, which examined the scandal at Mid Staffordshire 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.
Researchers 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 per cent 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 per cent since 2013, while there has been a 30 per cent 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 per cent of nursing staff in 2017 compared with 69 per cent in in 2013."
The researchers at the University of Southampton pointed to a 10 per cent average vacancy rate for registered nurses across the country, with some trusts reporting a 20 per cent rate.
And while nursing numbers have increased since the 2013 Mid Staffordshire 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."