A crackdown on "unacceptable" high payments from hospitals to potentially ill-qualified temporary bosses has been ordered by the health service's financial regulator.
It is understood NHS chiefs are paying as much as £2,000 a day, equivalent to £480,000 a year, to some stand-in staff.
In a letter to health chiefs, NHS Improvement warned that, on average, stand-in staff were earning twice the money of full-time employees and "this average hides many instances where the premium is much higher".
Its chief executive, Jim Mackey, wrote that the health service had "developed some bad habits" in issuing sizeable payouts without adequate justification.
He said: "This is unacceptable when in some cases there is not enough evidence of need--or indeed that the quality of leadership being 'purchased' is of sufficient quality--to justify such high pay rates."
Any contractors being paid more than £750 a day require the approval of the regulators, Mr Mackey wrote.
The letter also made a stark warning that pay arrangements could be exploited by some as "a method of tax avoidance".
It added: "This is deeply unpalatable to taxpayers, and to other NHS staff."
Analysis found interim hospital financial directors were earning twice as much as permanent staff at £1,800 a day, which is equal to £432,000 annually.
In a separate statement, Mr Mackey added: "The NHS has lots of talented and committed staff willing to take on important roles when organisations go through tough times.
"But for the sake of patients and staff, interim directors need to be demonstrably high quality. In other sectors, 'turnaround' is a profession and the skills and experience required for those tough jobs are recognised formally with accreditation.
"We will help providers reduce the amount of money they spend on interims and cracking down on off-payroll deals will help us get better value for money in the long run."