The NHS in England is spending tens of millions of pounds employing a few hundred agency staff, new figures show.
Over 500 agency staff are raking in at least £150,000 a year, including around 100 who each earn in excess of £200,000, according to NHS Improvement.
This means the health service is spending more than £80 million a year on 500 members of staff.
NHS Improvement said that greater transparency is needed around the pay for these workers.
In a letter to NHS provider bosses, the health watchdog suggested that trusts may be asked to publish data on how much their highest-earning locum staff are paid.
NHS Improvement's chief executive Jim Mackey wrote: "We currently have over 500 agency staff earning in excess of £150,000 per year per whole time equivalent.
"Around 100 of these individuals each earn in excess of £200,000 per year from agency work.
"We must all now work together to reduce this level of spend on any one individual.
"We believe that greater transparency is needed around the pay of these staff, and we will work with trusts to agree actions to achieve this; as part of this we may ask providers to publish rates paid to high-earning locums."
The letter also urges health leaders to crack down on NHS staff who are moonlighting at other organisations as agency workers to get "significantly higher rates" of pay.
NHS Improvement has written to hospital bosses to urge them to stop using agencies which use workers who are substantively employed at another NHS trust.
It said these staff are often getting "significantly higher rates for these additional hours".
"This is counterproductive for the NHS and cannot continue," the letter adds.
In 2015, health officials announced a package of measures to clamp down on agency spending, including caps on the hourly rates, after it emerged that the NHS in England spent more than £3 billion on agency workers in 2014/15.
NHS Improvement said that since the caps have been introduced more than £1 billion has been saved.
But there is still much to be done to reduce spending - particularly around the issue of agency and locum doctors, it said.
It said that £300 million per year could be saved if all medical locums charged rates within the set price cap.
A new national target has been set to reduce medical agency expenditure by £150 million in 2017/18.