According to one researcher, one in four nurses in the NHS are dangerously low numbers of nurses

Experts warned that a quarter of the NHS services were operating at such low staffing levels that patient safety was threatened.

The researchers found that a national shortage of nurses and the inability to increase their numbers sufficiently had not been resolved.

They pointed to a dilution of skills in hospital services, with medical badistants being used to strengthen the workforce.

Researchers at the University of Southampton also said that lessons learned from the Mid Staffordshire scandal had been somewhat lost due to a lack of staff investments and shortage of nurses .

According to a study conducted in 2016 by the university, the replacement of a single nurse by a less qualified staff member was badociated with a 21% increase in the risk of death.

The latest study indicates that the number of full-time equivalent nurses employed in NHS trusts has increased by 10% since 2013, while the number of health care badistants and support staff has increased by 30%.

"The disproportionate increase in support staff has resulted in a slight decrease in the skill mix," says the study.

"Registered nurses represent 66% of nurses in 2017, up from 69% in 2013."

Following Francis investigations, which examined the Mid Staffs scandal where negligence contributed to the deaths of patients, the National Institute of Health and Care Excellence (Nice) recommended that one number of eight patients per registered nurse triggers a staffing review.

But the latest study, which included an interview of 91 nursing directors, revealed that a quarter of the NHS services were working regularly at this unsafe level of staff.

The researchers pointed to an average vacancy rate of 10% for registered nurses, with some trusts reporting rates of up to 20%.

The number of nurses has increased since the 2013 mid-staff survey, but the growth in the number of patients means that there has been no improvement in levels Staffing.

Professor Jane Ball, lead author of the study, said, "One of the biggest problems has been the shortage of registered nurses at the national level.

"The current national shortage of AI and the inability to sufficiently increase the supply have not been resolved.

"This failure has prevented reaching safe staffing levels."

She said that a lack of investment meant that trusts had a clear view of staff safety but did not have sufficient means – in terms of the registered nurse – to achieve it.

Patricia Marquis, director of the Royal College of Nursing for England, said: "Now that there are 40,000 unfilled nursing positions in England, it is time for ministers and the NHS to master the situation before it does not deteriorate further.

"The Francis report's legacy was a unique opportunity to increase the numbers of nurses, but any short-term progress in hospitals has disappeared."

"The increase in the number of patients exceeds small increases in nurses."