The NHS spent £25 million on agency midwives last year - a figure that has doubled in two years, according to a new report.
The study, from the Royal College of Midwives (RCM), found a rapid rise in the spend on agency staff to plug rota gaps, as well as rising overtime costs and large spends on NHS bank staff.
The Government has tried to clamp down on spiralling agency costs in the NHS - which have helped fuel a £2.5 billion deficit - by introducing caps on how much each trust can pay agency staff.
But the new report found hospitals are relying heavily on agencies, paying an average of £41.25 per hour for midwives. Around half this cost goes straight to the agency.
The RCM sent Freedom of Information (FoI) requests to all NHS trusts in England with maternity units, of which 123 (92%) responded. Less than half of these (46) used agency staff in 2015 but accounted for a huge £24.95 million spend.
This was up on the £17.85 million in 2014 and the £11.75 million in 2013. From 2012 to 2015, the total spend on agency midwives was almost £65 million.
Between 2014 and 2015, there was a 40% rise in agency spending and a 146% rise over the four years.
The report showed that the highest average spend on agency staff across England was in December, at £50.58 per hour - after the Government introduced its pay cap.
A further £4.52 million was spent on overtime in 2015, while spending on bank staff - in-house NHS workers who want to work flexibly across health trusts - was £43.23 million.
Overtime spend on midwives costs the NHS about half as much as agency staff (£23.06 per hour), while bank staff cost £25.63 on average per hour.
The combined spend on agency midwives, overtime and bank midwives to the NHS reached £72.7 million in 2015.
The RCM argues this is enough money to pay for 2,063 full-time midwives with a decade of experience, or 3,318 full-time newly-qualified midwives.
Jon Skewes, director for policy, employment relations and communications at the RCM, said England currently has a shortage of 3,500 midwives and the money being spent would easily plug workforce gaps.
He added: "The findings of this report are deeply concerning and clearly reveal that many trusts within England are far too reliant on agency and bank midwives.
"This is an incredibly expensive and wasteful way to staff maternity units and it simply cannot continue. For over a decade now the RCM has warned that an over-reliance on temporary staff will inevitably cost more in the long run.
"There were 23 trusts that spent over £1 million on agency, bank and overtime and 12 of the highest spenders were in London. An over-reliance on temporary staff is clearly more expensive than employing the correct number of permanent staff and needs to be corrected sooner rather than later."
Mr Skewes said evidence showed that women have better outcomes when they see the same team of midwives.
"The way for trusts to provide this is to ensure their units are staffed correctly with the right numbers of permanent midwives rather than relying on temporary staff," he said.
The FoI found that of the 46 NHS trusts that used agency staff, 28 relied on them every month.
Eight trusts spent more than £1 million on agency staff alone in 2015, with the highest spending trust (£2.25 million) being West Hertfordshire Hospitals NHS Trust.
The average spend per trust was £542,394.
An NHS Improvement spokeswoman said: "This report shines a light on one of the ways in which overuse of expensive agency staff is presenting problems for the NHS.
"Over the course of 2015/16, the NHS was on course to spend around £4bn on agency staff, which is why we introduced the agency controls and price cap.
"Trusts have made good progress and saved over £600 million since last year; we are due to publish the latest figures on the agency controls early next week.
"We are committed to helping the NHS cut the cost of agency midwives and all agency staff, so that patients get the right care, from the right staff, at the right time."