Patients may face serious risks if they are cared for in hospitals where nurses have heavier workloads, a major new international study has found.

The study included over 422,000 patients in 300 hospitals throughout Europe, including Ireland, England, Belgium, Finland and the Netherlands. All were aged 50 or older and had undergone common surgeries, such as hip replacement, gall bladder surgery and appendectomy. The study also included over 26,000 nurses.

It found that patients are more likely to die after undergoing a common surgical procedure if they are cared for in a hospital with heavier nurse workloads.

The risk is also increased if there are fewer nurses with bachelor's degrees.

This marks the largest investigation of nursing and hospital outcomes in Europe ever undertaken. It found that for every extra patient added to a nurse's average workload, this increased the chance of surgical patients dying within one month of hospital admission by 7%.

On the other hand, a 10% increase in the number of nurses with a bachelor degree was associated with a 7% fall in the risk of death.

"Our findings emphasise the risk to patients that could emerge in response to nurse staffing cuts under recent austerity measures, and suggest that an increased emphasis on bachelor's education for nurses could reduce hospital deaths," said lead researcher, Prof Linda Aiken, of the University of Pennsylvania School of Nursing in the US.

The study noted that nurse workloads and education levels varied a lot between countries and between hospitals within a country.

Overall, it suggested that the highest risk of death after surgery in hospitals occurred in those where nurses with less education cared for more patients.

It found, for example, if nurses cared for an average of six patients each, the risk of death would be almost 30% lower if at least six in 10 of these nurses had bachelor's degree, compared to hospitals where nurses cared for an average of eight patients each and less than one-third had degrees.

"Our data suggest that a safe level of hospital nursing staff might help to reduce surgical mortality, and challenge the widely held view that nurses' experience is more important than their education," Prof Aiken added.

Details of these findings are published in the medical journal, The Lancet.