Agency doctors are up to£4,500 a year cheaper than locums from the NHS's own staffing bank, research claims.
Independent healthcare analyst Laing and Buisson compared the cost of employing doctors through commercial recruitment agencies with hiring from NHS Professionals (NHSP), the in-house agency used by more than 60 health trusts.
It found a locum from NHSP's bank of doctors costs on average£2.40 an hour more than one employed by a medical recruitment agency signed up to the National Framework Agreement.
This works out about£4,500 a year more expensive. But the report suggests the cost difference could spiral to£10,000 should NHSP increases charges to rid itself of debt.
The report said agency fees included fewer extra costs and they had much higher success rates for filling doctor shifts.
The Association of Medical Recruitment Agencies (AMRA), which commissioned the research, also argued its workforce was subject to more rigorous safety checks under regulation by the NHS Purchasing and Supplies Agency (PASA), a claim denied by NHSP.
"People assume agency staff are lower quality and higher cost and this report very clearly refutes that," said AMRA executive committee member Kate Bleasdale.
An NHSP spokesman said locum doctors were only a fraction of the range of NHS staff it places, representing just£10 million of its£235 million turnover.
He added that the prices quoted in the report included discounts so were not as clear-cut as the report suggests.
"We accept this small part of the business is loss-making. However, this is not a structural issue and we have brought in a new management team to turn the business around."
A DoH spokesman added: "We are working with NHSP to improve efficiency and bring down rates to at or below market prices."
Meanwhile new figures published by the Information Centre for Health and Social Care show vacancy rates in the NHS are at an all-time low.
However, unions said the statistics masked the reality of job cuts and freezes.