The NHS is being left to "pick up the pieces" after poor surgery in privately-run but publicly-funded treatment centres, experts are warning.
Private centres threaten NHS hospitals' survival, it is claimed.
Surgeons are carrying out procedures for which they are not trained, leading to a high rate of complications, says the British Orthopaedic Association.
In the British Medical Journal, top surgeon Professor Angus Wallace also warned of high error rates.
The Department of Health said it was confident standards matched the NHS.
Public Health Minister Lord Warner said 250,000 patients had received operations or scans faster because of the opening up of the private centres.
The development of partnerships with the private sector was one of former health secretary Alan Milburn's flagship measures to reduce waiting lists.
But the BMJ article argues that although the expansion of private treatment centres - or ISTCs - has led to more operations being carried out sooner, it has occurred at a price.
Professor Angus Wallace, of Nottingham University's Medical School, said the complication rate at ISTCs was up to ten times the norm.
He said: "The number of patients we are seeing with problems resulting from poor surgery - incorrectly inserted prostheses, technical errors, and infected joint replacements - is too great."
Surgeons from overseas have been flown over to increase the numbers of operations like joint replacements being carried out, he said.
"They have come from many countries, usually European, and their training and clinical practice at home are quite different from those to which they are exposed in ISTC."
His comments are mirrored by surgeons' body the British Orthopaedic Association, which warned a Commons health committee looking into private treatment centres about a string of problems associated with them.
Association Chairman Ian Leslie said: "The quality assurance of the surgeons is not robust and allows the opportunity for surgeons to take on operative procedures for which they are not trained."
He added that NHS surgeons, especially in major joint replacement centres, were seeing "above average revision rates and re-admission rates for ISTCs".
He warned that not only were NHS surgeons being left to "pick up the pieces" but if surgery did not give the right results patients had no choice but to complain to their GP, who had no way of sending them back to the treatment centre.
President of the Royal College of Surgeons of England Bernard Ribeiro told the committee the government had "lost the plot" when it devised the policy.
He said private treatment centres were threatening the very survival of NHS hospitals over the long term as more and more elective surgery was taken away from them.
"By throwing all this money into ISTCs it is challenging the NHS and it will go down," he warned.
He gave the example of a hospital in Greater Manchester where two of the wards had been closed as a result of contracted work going to ISTCs.
Mr Ribeiro called for an audit of the work being carried out by private centres to see if it was equivalent to that being done by the NHS.
Health Minister Lord Warner said all independent sector providers had to meet the same standards as NHS staff.
Patient safety is the paramount concern and clinical data is audited by independent experts, he said.
"All facilities that operate on patients are inspected by the Healthcare Commission before they open and can be inspected without notice at any time thereafter.
"Any provider who breaches the contracted clinical standards or recruitment processes can be penalised and ultimately risks termination of their contract," he added.