A report into the controversial Independent Sector Treatment Centres (ISTCs) programme has found more than half are failing to provide the correct information as stipulated in their contracts.
It means patients are unable to compare NHS treatment in private clinics because of poor government planning and a lack of comparable data, the study added.
Healthcare Commission (HCC) chief executive Anna Walker blamed the Department of Health for not ensuring ISTCs submit performance figures that can be compared with NHS hospitals.
But she said she believes it is more of a "cock-up" than a conspiracy by officials to stop the results being analysed.
While patients attending ISTCs reported high levels of satisfaction, the commission's report highlighted a number of problems.
There are currently 24 ISTCs in England but five out of the 12 that were assessed are still carrying out less work than they are contracted to do.
Private firms have contracts with the NHS to run the clinics, which offer non-emergency services and procedures such as cataract operations, and MRI scans.
Karen Jennings, Unison head of health, said: "Millions of pounds have been pumped into these private treatment centres so you would expect patient satisfaction to be high.
"However, the devil is in the detail - the report highlights the fact that some ISTC contracts do not reflect local needs and are not flexible enough to be changed as service needs change."
But health minister Ben Bradshaw said: "ISTCs have the same responsibility as the NHS to provide high quality care to patients and we are pleased that the HCC report shows that they are performing well in a number of areas and are rated highly by patients."
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