Patients are being put at higher risk of dying in hospitals where lower staffing levels of registered nurses are leading to incomplete care, a new study has found.
Jane Ball, principal research fellow at the University of Southampton, has found that lack of time is the "missing link" in understanding why mortality rates vary between hospitals.
The research, published in the International Journal of Nursing Studies, shows that when registered nursing (RN) levels are lower, necessary care is more likely to be missed.
Each 10% increase in the amount of care left undone was associated with a 16% increase in the likelihood of a patient dying following common surgery, the study shows.
Dr Ball said: "For years we have known that there is a relationship between nurse staffing levels and hospital variation in mortality rates but we have not had a good explanation as to how or why.
"These results give the clearest indication yet that RN (registered nurse) staffing levels are not just associated with patient mortality, but that the relationship may be causal.
"If there are not enough registered nurses on hospital wards, necessary care is left undone, and people's lives are put at risk."
The findings come from further investigation of the major RN4CAST study of nurse staffing at hospitals in nine European countries, including 31 NHS acute trusts in England.
Previous analysis of the survey showed that lower nurse staffing levels were associated with higher mortality.
Missed nursing care was measured through a nurse survey and included activities such as patient surveillance, administering medicine on time, adequate documentation, comforting patients and pain management.
The analysis also looked at nurse qualification and confirmed that hospitals with higher numbers of registered nurses trained at degree level had lower risk of patient mortality.
Professor Peter Griffiths, chairman of health services research at the University of Southampton, said: "This study reinforces the importance of registered nurses who are trained to a degree level.
"It is more evidence that shows that you cannot substitute fully qualified RNs with less qualified staff, without taking a risk with patient safety.
"It is the number of RNs on duty that is key to ensuring complete care and minimising the risk of patients dying."
Co-author Luk Bruyneel, of the University of Leuven in Belgium, said: "These findings have implications for healthcare managers and policymakers.
"Monitoring missed care may offer a more responsive and sensitive early-warning system for hospitals to detect problems before patients die.
"More work needs to be done worldwide to ensure we utilise this data for the benefit of patients."
Janet Davies, chief executive and general secretary of the Royal College of Nursing, called on the Government to take note of the research and provide funds to recruit more nurses.
She said: "Despite years of warnings, hospitals across the country do not have enough nurses. This research puts beyond doubt that patients pay the very highest price when the Government permits nursing on the cheap.
"As the nurse shortage bites, hospitals are filling wards with unregistered healthcare assistants in a bid to cope, especially at night. Ministers cannot ignore further evidence that the lack of registered nurses leads to people left in pain for longer and a higher risk of not recovering at all.
"Pressure in the NHS is mounting, nurses are pulled in every direction and important things are inevitably missed. Patients stand a better chance of recovery when there are more degree-trained nurses on the wards.
"The Government must redouble its efforts to train and recruit more nurses and stop haemorrhaging experienced professionals who feel burnt out and undervalued. In the forthcoming budget, the Chancellor must scrap the public sector pay cap that stands in the way."
Unison head of health Sara Gorton said: "Yet another study has shown that plummeting staffing levels and unrealistic workloads are having a direct impact on patient mortality rates.
"Nobody should have basic care denied to them at their most vulnerable time of need, or be given so much work that they are unable to perform their duties properly.
"The study also reveals that wards with nurses who are trained to degree level had a lower risk of patient mortality and offered a simple solution to the findings, more registered nurses on duty.
"Unfortunately, the removal of nursing bursaries, chronic staff shortages and the 1% pay cap mean that more nurses are currently leaving the profession than joining.
"As we have said in the past, the Government urgently need to change its policies, putting safety before savings."
A Department of Health spokeswoman said: "We expect all parts of the NHS to make sure they have the right staff, in the right place, at the right time in order to provide safe care.
"This is why there are over 31,100 more clinical staff since May 2010 - including 11,983 more nurses on our wards - and we have recently announced further funding to train an additional 10,000 nurses, midwives and allied health professionals."