The NHS is paying agency doctors millions of pounds every year - with one having charged £363 an hour, according to a health watchdog.
Data from NHS Improvement shows that 20 of the most expensive locums cost the NHS £7.5 million - or an average of £375,000 each - a year.
High rates are not only paid to those working full time as locums.
Reports to the watchdog show that one doctor has charged £363 an hour, while a further five doctors were found to be charging more than £250 an hour for working limited sessions within trusts.
The regulator said recent measures have aimed to tackle situations where trusts have used agency staff for lengthy periods of time.
But it added that reports have shown that there are more than 300 locum doctors who have been working at the same trust for more than two years.
NHS Improvement analysis has shown that £51.5 million could be saved every year if medical and dental agency shifts paid over the cap were reduced by £5 an hour, or even £103 million if they were reduced by £10 an hour.
It also shows that £300 million a year could be saved if medical locums charged rates within the set price cap.
The watchdog said that rates currently being charged to hospitals were on average 28% above the rates that were set, adding that if locums charged within the rate, the savings made could be used to pay for an extra 30 consultant shifts a week for each trust in the country.
NHS Improvement has also heard of a select group of around 260 high-cost doctors whose rates are accounting for a significant part of the agency costs - with around 260 doctors each charging the NHS more than £240,000 a year.
A spokeswoman for the watchdog said: "NHS Improvement and the Department of Health are currently developing a new range of tough measures which will tighten the agency controls around very highly paid medical locums, details of which will be announced."
Last month, a report into five-yearly checks on doctors for the General Medical Council (GMC) found some locums were falling through the gaps when it came to having their fitness to practise assessed.
The study warned that information on locum doctors was not always shared between the hospitals where they worked, while some hospitals were "unwilling to provide frank feedback" when a doctor was not up to par.
Sometimes hospitals told agencies, which make millions of pounds from supplying temporary doctors, "Please don't send this doctor again", but did not give full reasons why.
While monitoring of locum doctors was better than it used to be, there were still "weak" parts of the system, the report said.
There are 86 locum agencies across England, covering 8,517 doctors.