Two fifths of private hospitals in England are failing to meet expected safety standards, according to a report by health inspectors.
While the majority of independent acute hospitals provide good quality care to patients, inspections by the Care Quality Commission (CQC) raised concerns over the safety and leadership of some services.
The regulator also said "a lack of effective oversight" of consultants with practising privileges was a "major concern".
The issue had been "brought into sharp focus" by the case of rogue breast surgeon Ian Paterson, who carried out unnecessary operations in NHS and private hospitals, it added.
The Royal College of Surgeons (RCS) said the report "exposes the poorer practices of some independent providers and underlines the need for a renewed focus on improving patient safety".
Of the 206 independent acute hospitals inspected, 62% were given a "good" rating overall and 8% were described as "outstanding", according to the report.
However 30% of private hospitals were deemed to require improvement.
The CQC said it was particularly concerned about safety, with 41% of private hospitals rated as requiring improvement in this area and 1% as inadequate.
Meanwhile, almost a third (30%) were rated as requiring improvement and 3% as inadequate in terms of how well they were led.
In his foreword, Professor Ted Baker (pictured), chief inspector of hospitals at the CQC, wrote: "Our inspections also identified concerns around the safety and leadership of some services, often as a result of a lack of safety checks and poor monitoring of risks.
"Too often, safety was viewed as the responsibility of individual clinicians, rather than a corporate responsibility supported by formal governance processes.
"In particular, we found that monitoring of medical governance such as scope of practice of individual consultants was not consistently robust.
"Such a failure of effective governance was brought into sharp focus with the recent case of the surgeon Ian Paterson."
Consultants operating under practising privileges are not employees of a hospital but regulation sets out that they should be considered in the same way as other staff.
In some cases the CQC found a "lack of robust and effective oversight of practising privileges", with many consultants treated "as 'customers' bringing business to the hospital" meaning providers "could be reluctant to challenge them".
"It is essential that providers demonstrate that they are proactively auditing and monitoring consultants' work, and have a real oversight of services in order to protect patients and ensure they are being treated safely and effectively," the report said.
"In many cases they could not."
The CQC report also said "informal practices and consultant behaviour", including some instances of medics failing to follow the World Health Organisation surgical checklist, were risking safety in the operating theatre.
Professor Derek Alderson, president of the RCS, said: "The recent Ian Paterson case demonstrated that there is no room for complacency and further actions should be taken to minimise harm to patients in both the NHS and private sector.
"We must continue to promote a culture, in all types of healthcare, where patient safety concerns are not brushed aside."
He also called for the private sector to be required to report similar safety and quality data as the NHS, for example on unexpected deaths.
The report provides the first comprehensive analysis of the quality of care provided by independent acute hospitals since the CQC introduced an inspection programme in 2015.
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