NHS trusts in England spent more than £1.4 billion last year on temporary nursing staff due to shortages, a study has found.
The bill was enough to pay the salaries of an estimated 66,000 newly qualified nurses, and more than fill the 38,000 existing vacancies in nursing jobs, a report by The Open University claims.
Temporary agency and bank nurses were brought in to plug the gaps in staffing for a total of 79 million hours last year - and were paid 61% above the hourly rate of a newly qualified nurse.
Estimates suggest if current vacancies were filled permanently, the NHS could save as much as £560 million a year.
The figures were released under the Freedom of Information Act - with only 146 of the 241 hospital trusts providing data, meaning the total bill spent on temporary nurses could be much higher.
Jan Draper, professor of nursing at The Open University, said relying on temporary nurses was a "sticking a plaster over the problem" and cost "considerably more" than filling vacancies permanently.
She said: "We know that poor retention and low recruitment results in inefficiencies and ultimately puts patient care at risk, so it's vital that we look to a more strategic and sustainable approach.
"Taking advantage of recently introduced degree apprenticeships that offer flexible work-based learning is one solution, making use of funding already ring-fenced to pay for training while opening up new routes into the profession."
Janet Davies, chief executive and general secretary of the Royal College of Nursing, said the figures exposed the "utter false economy in current NHS staffing".
She added: "Short-sightedness in recent years has left tens of thousands of unfilled nurse jobs, to the severe detriment of patient care.
"Workforce planning has been ineffective and dictated by the state of finances, not the needs of patients.
"It is further proof that cost-cutting plans saved no money at all and, instead, increased agency costs, recruitment fees and the sickness absence bill through rising stress.
"Ministers should look at these figures and demonstrate they have the political will to fix the shortages."
The news comes after a survey revealed most people believe there are not enough NHS nurses to keep patients safe.
Nearly three quarters (74%) of UK adults believe there are too few to provide adequate care, according to the poll by the Royal College of Nursing (RCN).