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.
Under Health Education England's latest workforce plan, the number of nurses in training is increasing by 13,748 during this decade.
A Department of Health spokesperson said: "Patient safety is top of our agenda, and so in the wake of Mid Staffs, agency workers have been used to correct historic understaffing.
"Since May 2010, we already have nearly 8,000 more nurses on our wards.
"We want to reduce reliance on agency staff in the longer term, and we committed in the recent pay deal to work with the unions to bring the bill down."
Earlier this week, the Commons Public Accounts Committee warned about the rising cost of temporary medical staff.
Its report recommended that the Department of Health make better use of the NHS' position as the dominant employer.
Labour has pledged to train 10,000 more nurses across the next Parliament.
The shadow health secretary, Andy Burnham, said: "Nurses who've had a pay freeze for years will find it galling that the agency bill is going through the roof.
"The over-reliance on agency staff is not just wasteful of NHS finances, but it can also damage staff morale and result in poorer patient care."