One in four NHS wards routinely operates at staffing levels so low that patient safety is threatened, experts have warned.
Researchers have found that a national shortage of nurses and a failure to increase their numbers sufficiently has not been addressed.
They pointed to a dilution in skills on hospital wards as healthcare assistants were being used to shore up staff numbers.
The University of Southampton researchers also said lessons learnt from the Mid Staffordshire scandal had been somewhat lost because of a lack of investment in staffing and the nurse shortages.
A 2016 study by the university found that, for every 25 patients, substituting just one nurse with a lower qualified member of staff was linked with a 21 per cent rise in the odds of patients dying.
The latest study said the number of full-time-equivalent nurses employed in NHS trusts had increased by 10 per cent since 2013, while numbers of healthcare assistants and support staff had grown by 30 per cent.
“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.”
Following the Francis Inquiries, which examined the scandal at Mid Staffs where neglect contributed to patient deaths, the National Institute for Health and Care Excellence (Nice) recommended that a level of eight patients per registered nurse trigger a review of staffing.
But the latest study, which included questioning 91 directors of nursing, found a quarter of NHS wards regularly worked at this unsafe staffing level.
The researchers pointed to an average vacancy rate of 10 per cent for registered nurses, with some trusts reporting rates of up to 20 per cent.
Nurse numbers have increased since the 2013 Mid Staffs inquiry, but 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.
“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.”
She said a lack of investment 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: “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 but any short-term progress in hospitals has fallen away.
“Rising patient numbers are outstripping small nurse increases.”