The health secretary, Alan Milburn, has moved to calm the fears of health workers, saying that excessive private sector involvement in the NHS would irreparably damage the service.

In a speech to the NHS Confederation, Milburn said there would be a new relationship with the private sector but stressed that it would be limited. "We risk the ethos of the NHS - its values and its principles - at our peril. That is why, when we say we will forge a new relationship with the private sector, it is just that. A new relationship, not a takeover," he said.

He praised those who work in the service and warned that too much private sector involvement would be damaging. He said the government was not intent on "creating a mixed economy of care" saying he wanted to ensure "it is about maximising the care that is available to NHS patients based on NHS principles".

"People do not work in the NHS to make a mint. They work in the NHS to make life better for others. It is the ethos of public service. It is the burning ambition to serve people regardless of their wealth or worth that lies at the heart of public support for the NHS. Just as surely as that ethos of public service makes the NHS, losing it would break the NHS," he said.

Whilst he admitted that the profit motive was unacceptable in healthcare, he warned that it would be wrong to think there was no role for the private sector.

"The free market in health care doesn't work. It is not the right way forward for health care in Britain. While some subscribe to the philosophy of all things private are good, all things public are bad, I say that that philosophy belongs not to today, but yesterday," he said.

His comments came amid widespread criticism of the government's plans for unions and Labour backbenchers.

Labour MP Kevan Jones warned the government that it had got its language wrong on the issue.

"The problem I think is the use of language which I think has lead people to think the private sector has to be brought in at any cost. My fear about that is that that will send a clear signal to civil servants who will than go full steam ahead ignoring both public opinion and also backbench parliamentary opinion," he said on Friday.