NHS spending on agency nurses has soared in the past two years, says the Royal College of Nursing (RCN).
Its head said a “payday loans” attitude to workforce planning had emerged.
The trade union said the figure had risen from £327m in 2012-13 to £485m last year in 168 trusts in England.
The Department of Health said agency workers had been used to correct “historic understaffing” – but it wanted to reduce reliance on these staff in the longer term.
The RCN received responses from 73% of acute, community, mental health and specialist NHS trusts in England to a request about spending on agency nurses, under the Freedom of Information Act.
It said the figure for those 168 trusts could reach £714m by April, based on data from the first two quarters of this financial year.
The union estimates the overall bill could soon reach £980m a year, if it goes on rising at a similar rate.
It found half of the trusts who responded were spending double on agency nurses last summer than they were two years earlier.
And only 19 of the trusts were spending less on agency nurses than in 2012/3.
Dr Peter Carter, Chief Executive & General Secretary of the RCN, said: “This report shows the true financial cost of a health service which takes a ‘payday loans’ attitude towards workforce planning.
“What it doesn’t show is the cost to patients. Over-reliance on agency staff is bad for continuity of care, and that is bad for patients.
“The NHS is under immense pressure and it is now time for serious workforce investment and sensible, long-term workforce planning.”
The RCN believes the rise in agency spending is partly due to trusts increasing staffing levels in the wake of winter pressures and the Francis report into the Mid Staffs scandal.
It says there are supply pressures too, with NHS pay restraint making recruitment and retention more difficult, and leading more nurses into agency work.