According to the union, spending has increased by 75.7% over two years between 2012 and 2014.
The £17 million the NHS spent on agency midwives is equivalent to hiring 115 full-time midwives.
Spending on agency staff has varied across 136 trusts in England, with one spending just £358 on agency staff between 2012 and 2014.
However, another trust spent £4,482,432 over the same period.
The figures were obtained by the Royal College of Midwives under the Freedom of Information Act.
Around 43 out of 130 trusts who responded to the request said they had used agency staff at some point over the last three years.
Responding to the findings, Director for Policy, Employment Relations and Communication at the RCM, Jon Skewes said: “These findings show that some trusts in England are heavily reliant on agency midwives to ensure they can provide a safe service and these costs are spiralling out of control year after year. This is an incredibly expensive and wasteful way to staff maternity units especially because over half the spend per hour that the trusts are paying is not going to the midwives but are on-costs going to the agencies.
“We have found that many midwives who chose to work for an agency did so because they were denied the right to work part-time or flexibly. In turn we have a ridiculous situation when midwives leave an organisation because they can’t work flexibly and then are employed by the same trust as an agency midwife. This results in the trust paying 2.7 times more for that midwife, and over half of that goes to the agency.”
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