Fast-track surgery centres are treating people up to eight times quicker than other NHS providers, a government report says.
Mobile centres treated 10,000 cataract patients in 11 months
More than 120,000 patients have been seen at the 30 treatment centres and two mobile units since 2002.
Ministers said the centres, which provide streamlined diagnosis and surgery, have helped drive down waiting lists to record levels.
But doctors expressed concern the units undermined the work of hospitals.
Hospital equipment has been known to be left unused while mobile treatment centres carry out health care nearby.
The centres carry out non-urgent surgery on NHS patients, such as knee, hip and cataract operations.
Dr Paul Miller, chairman of the consultant's committee of the British Medical Association, said it was good that waiting lists were down.
However, he said: "The treatment centres that have been brought in have caused a number of problems that he have warned the government about.
"In particular, we warned that these treatment centres would destabilise parts of the NHS, and lead to closures of wards, operating theatres and job losses, and this is now happening in some NHS hospitals.
"And we also warned that if they were given more favourable funding than the NHS, and they were allowed to cherry pick easy and profitable procedures that this would be to the detriment of the NHS - and this is also now happening."
Dr Miller said the treatment centres also failed to provide opportunities to train future consultants.
He denied that the BMA was opposed to the centres because they reduced the opportunity for doctors to boost their income through private practice.
But Health Secretary John Reid defended the centres, saying they have meant more staff and more capacity within the NHS.
"We are demanding change but nobody can say it is destabilising."
He said they have "contributed to the lowest waiting list figures since records began".
Figures released on Friday showed in November last year the waiting list stood at 844,000 - a fall of 13,000 since October and the lowest since data was first collected in September 1987.
Mr Reid said: "Treatment centres play an important role in speeding up access to treatment for patients and improving quality of care and patients' experience.
"This is shown by the mobile cataract chain treating 10,000 patients in less than 11 months - operating at a rate almost eight time faster than traditional NHS services.
"This higher rate is achievable because the units are able to concentrate on a single procedure in a modern, purpose built unit."
Shadow Health Secretary Andrew Lansley said: "Unfortunately, the government's central target for the procurement of elective surgery through independent treatment centres is just one recent example of how the government continues to interfere in the management of the health service."
And he added patients needed choice about "where and when" they received treatment rather than being dictated to by ministers.
While Liberal Democrat health spokesman Paul Burstow said: "The test for treatment centres is whether they increase the NHS ability to treat more people.
"If the centres simply divert operations from existing NHS facilities and increase costs they will have failed. "
Another 17 NHS-run centres are opening in 2005 and 34 private sector units are also planned.