Cataract surgery is one of the operations carried out by treatment centres

Health trusts chief executives also said they were being bullied into accepting them by ministers in a survey by the Health Service Journal.

Eight independent fast-track centres, which carry out non-urgent surgery on NHS patients, have been set up so far but another 34 are in the pipeline.

Last week ministers hailed the success of the units in cutting waiting lists.

We have been concerned about reports about activity being stripped out of existing NHS providers in order to meet the target Nigel Edwards

The majority of treatment centres currently operating are NHS ones but the government is looking to roll out more units provided by the independent sector as they look to get the private sector to carry out 15% of NHS surgery by 2008.

Government figures showed that 120,000 people have been seen at the centres, which normally carry out simple procedures such as knee, hip and cataract operations, since they were first set up in 2002.

Some 16,000 of these were carried out at the eight private units.

At the time, Health Secretary John Reid said the units were up to eight-times faster than the NHS.

But a survey of 111 acute and primary care trust chief executives, about a quarter of the total, revealed deep unease at the way independent treatment centres had been introduced.

Money

Eight in 10 said their trusts had been forced to reduce their activity, and three quarters complained the centres were not value for money.

Some 61% said the Department of Health's approach was prescriptive, while 37% said it amounted to bullying.

Nigel Edwards, policy director of the NHS Confederation, which represents health service managers, said he supported the use of the independent sector in the NHS but criticised the way independent treatment centres had been handled and the effect of the 15% target.

"We have been concerned about reports about activity being stripped out of existing NHS providers in order to meet the target."

And he added that it was unfair foundation trusts were barred from bidding for work being carried out by the independent treatment centres.

Health Minister John Hutton defended the centres, saying they were benefiting patients.

"I make no apology for procuring additional activity from the independent sector as it is helping to increase choice for patients and speed up waiting times, contributing to the lowest waiting list figure since comparable records began in September 1987."