The cost of drafting in agency staff to cover gaps in Northern Ireland's health service has topped £200m in the last year.
Spending on temporary workers has risen by over 160% since 2015.
At the current rate - around £640,000 a day - the annual bill will rise to more than £230m by April.
A leading health union warned that the costs were out of control.
It comes at a time of unprecedented crisis in the health system, with rising waiting lists and financial pressures.
Yesterday it emerged that some non-emergency operations have had to be suspended due to staff shortages.
Nurses are also set to go on strike next month over staff numbers and pay.
According to the latest Department of Health figures, the number of vacancies across the system totals more than 7,000.
This includes a shortage of almost 3,000 nurses and midwives.
To fill the gaps, the health service has become increasingly reliant on agency staff.
In the 12 months to April this year, the cost of temporary workers totalled £201,298,475 - £550,000 a day on average.
The cost has increased year on year, rising from £76,508,610 in 2014/15.
In the first three months of the current financial year, the cost was £58,043,621 - equivalent to £637,842 every day.
If it continues at this rate, the 2019/20 bill will be £232,812,000.
The Department of Health said agency staff were needed to help keep wards and other health facilities open.
However, Anne Speed, head of bargaining and representation at health workers' union Unison, said the expenditure was unsustainable.
''It's clear the cost of agency spending is spiralling out of control," she said.
"Public money is being wasted on high-cost private agencies to plug gaps in staffing, money which must be returned for investment in fair pay and the recruitment and retention of permanent staff.
"It must be abundantly clear to the Department of Health that this is not sustainable.
"They must come back to trade unions with a proper strategy that will deal with the staffing crisis in the short-term and solve the systemic problems in the long-term.''
Details of the costs of agency staff across the health service were released after a Freedom of Information request by this newspaper.
According to the 2018/19 figures, the biggest spend was on medical and dental costs, totalling £86.7m.
Nursing and midwifery agency fees totalled £52m - more than doubling in three years since the 2016/17 spend of £23.5m.
The latest Department of Heath workforce bulletin shows that, as of June, there were 2,972 nursing and midwifery vacancies here.
Pat Cullen, director of the Royal College of Nursing in Northern Ireland, said: "Agency costs have risen dramatically over recent years because of a serious nursing shortage across hospital and community services.
"This situation has been created over a number of years as a result of inadequate workforce planning and short-term cost-savings measures including reductions in the number nurses trained here. Currently, there are nearly 3,000 nursing vacancies within the health and social care sector and probably a similar number in the independent sector.
"As a result, the cost of deploying nursing staff via nursing agencies has reached unprecedented levels."
Ms Cullen said nurses are working unpaid hours to care for patients and many have voiced concerns about staff shortages.
She added: "This is one of the core reasons the Royal College of Nursing is now in a position where our members have decided that enough is enough and the only option open to them is to take industrial action and strike action."
Ms Cullen warned the situation was now critical, and should have been avoided.
"Pressures within the system have led many nurses to make career decisions to leave full-time employment and work for nurse banks or agencies, or to leave the profession altogether," she added.
"While the current levels of expenditure on agency staff are completely unacceptable, the sad reality is that they are plugging gaps which should never have been allowed to develop."
A spokesperson for the Department of Health said: "The Department is committed to growing our health and social care workforce, fill vacancies and reduce agency spend.
"There is a global shortage of nurses, linked to the fact that health care needs are increasing and the role of nurses expanding.
"Significant ongoing investment is required to build our nursing workforce and reduce the need for agency use.
"The department's commitment to addressing this situation is evident. The number of student nurses and midwives going through Northern Ireland universities is now at a record level of 1,025. It was 710 in 2015/16, which means it has been increased by 44% at a time of serious budgetary pressures.
"In the present situation, it is misleading for anyone to claim that agency use can simply be stopped overnight. Agency staff currently help keep wards and other health facilities open.
"It is also untrue to say that agency expenditure could easily and quickly be transferred to increase health service pay levels."
Earlier this month a Westminster committee warned that health services in Northern Ireland risk "deteriorating to the point of collapse" without a long-term funding strategy to support transformation.
In a hard-hitting report, the Northern Ireland Affairs Committee said services are struggling to meet the needs of an ageing population.
Committee chair Simon Hoare said the health service here was falling behind the rest of the UK.