Health service spending on private providers has almost quadrupled in the past five years, with the total during the period surpassing £150million.
Northern Ireland's health trusts spent £59million on private hospitals and clinics during 2021-22 compared to £16.79million in 2017-18, new figures show.
Belfast Trust spent the most over the five-year period at more than £68million, including more than £22million in the most recent financial year alone.
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People Before Profit's Gerry Carroll hit out at the spending, which he uncovered through a Freedom of Information request.
The West Belfast MLA said: "Our health service is at breaking point and yet huge sums are being handed to the private healthcare sector.
"These figures are a slap in the face for health workers, who are having to strike for fair pay and investment in services.
"The HSC (Health and Social Care) is paying private companies to carry out consultations and procedures that can and should be delivered in our own hospitals."
In total more than £151million was spent by Northern Ireland's health trusts on private healthcare providers between 2017-18 and 2021-22.
Belfast Trust spent the most at £68.03million, followed by the South Eastern Trust at £36.67million, the Southern Trust at £20.46million, the Western Trust at £13.39million and the Northern Trust at £12.88million.
The spending was £16.8million in 2017-18, £24.23million in 2018-19, £24.22million in 2019-20, £27.18million in 2020-21 and £59million in 2021-22.
Mr Carroll accused the Conservative government and Stormont's devolved administration of "driving patients and staff towards the private health sector" by not properly investing in the NHS.
He added: "The crisis in our NHS is easily solved with political and financial commitment.
"The money is there, but the political will is missing. Striking health workers will have to force the government's hand."
In a statement Stormont's Department of Health said the private sector has provided "vital additional capacity" when the health service has been under "severe and sustained pressure".
A spokeswoman said: "It is widely accepted that there is a serious shortfall between demand for health care in NI and the current capacity of our health service to meet that demand.
"Our appalling waiting lists are one of the most serious consequences of that mismatch.
"The health service has been mitigating the shortfall by paying for some patients to have treatments in privately run facilities.
"This investment is adding to available capacity. Without it, fewer people would receive the treatment they need.
"During the height of the pandemic, with our health service under severe and sustained pressure, the private sector provided vital additional capacity."
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