Figures reveal "wasteful" NHS trusts spent more than £17 million on agency midwives in one year - enough to employ 511 full time staff.
Obtained by the Royal College of Midwives (RCM) through Freedom of Information requests, the findings from a three-year period are published in a new report.
It says spending rocketed by 75.7% in two years - rising from £10,159,099 in 2012 to £17,849,767 in 2014 - the equivalent in wage costs to 511 midwives.
And the variation in the amount of agency spending between the 136 trusts in England is stark.
While one trust spent just £358 on agency staff between 2012 and 2014, another paid out £4,482,432 in the same period.
More than 11 trusts spent in excess of £1 million on agency staff over three years.
The RCM says staff shortages should be eliminated by employing more midwives and incentivising existing staff to work bank shifts and overtime.
"At present the cost of overtime is being controlled but agency spend, which is much more expensive, is less controlled. This needs to be corrected as current practice in the NHS is wasteful," the report says.
More than 43 of the 130 trusts that responded to the FOI request say they have used agency staff at some point in the last three years.
A qualified midwife with 10 years experience working full time in the NHS in England currently earns on average £34,876 a year - equating to £17.84 an hour.
If a midwife works overtime, past 37.5 hours a week, this rate would increase to £26.76 per hour.
Trusts had an average spend of £49.01 per hour on agency staff in 2014, while RCM members who work agency shifts reported a wage of just £22.84 per hour - showing half the cash goes on agency fees and other costs.
The average spend figure of nearly £50 equates to more than 2.7 times the hourly amount of a permanently employed midwife and more than 1.8 times the amount of overtime costs.
The report says: "It is well known that the use of agency staff in the NHS has reached inappropriate levels. Improving productivity is becoming a pivotal issue in the NHS.
"Not least because productivity is a key issue across the economy but because of the significant funding challenges facing the NHS combined with an increased demand for services due to the increasing birth rate and complexity of cases."
The report says most midwives who work agency shifts do so in addition to their permanent posts to top up their income, while others choose to work solely for an agency because of the flexible shifts.
Shadow public health minister Andrew Gwynne said: "David Cameron promised to recruit 3,000 more midwives, but he broke that promise and now spending on expensive agency staff is spiralling out of control.
"Across England, too many maternity units are operating without enough staff, forcing some to turn women away because they are unable to cope.
"Tory ministers cannot keep ignoring these warning signs. They need to set out how they intend to tackle staff shortages in the NHS and ensure expectant mothers get the care and support they need."
A Department of Health spokesperson said on November 23 last year, a shift rate cap for NHS staff was introduced to limit the amount companies can charge per shift for all staff.
"These figures cover previous years and do not reflect the current situation," said the spokesperson.
"For too long staffing agencies were able to charge hospitals extortionate hourly rates but the tough new controls we introduced last year are helping hospitals clamp down on agency staff, improving continuity of care for patients and will reduce the overall agency staff pay bill by £1 billion over the next three years.
"We want the NHS to be one of the safest places in the world to have a baby and there are already more than 5,500 obstetricians and gynaecologists in the NHS, including over 2,000 consultants, an increase of 20% since 2010."