Nick Clegg has pledged to not let the NHS be "flogged off to the highest bidder" as he set out substantial revisions to the health and social care bill.
The deputy prime minister seeked to maximise the Liberal Democrats influence over imminent changes to the government's NHS plans, due to be published within weeks.
Many of his party are unhappy at what they see as a move towards privatisation of the health service.
In a speech at a London hospital, the Lib Dem leader said he supports the use of private providers in the health care service and that they have improved patient choice.
However, he added: "It's not the same as turning this treasured public service into a competition-driven, dog-eat-dog market where the NHS is flogged off to the highest bidder."
Clegg highlighted the step taken by the government to "pause, stop and listen" in the progress of the legislation amid political and professional hostility.
The deputy prime minister said there was no question of the NHS no longer being free at the point of delivery.
"No bills, no credit cards, no worries about money when you're worrying about your health. That's why I have been absolutely clear: there will be no privatisation of the NHS," he said.
"The NHS has always benefited from a mix of providers - from the private sector, charities and social enterprises. And that should continue."
In a speech to the Royal Society of Medicine, shadow health secretary John Healey accused the prime minister of refusing to amend the bill enough to ensure that it does not harm healthcare.
He said: "David Cameron is a PR man looking for a PR answer. He must accept the problem is not the presentation of his NHS plans but the full-blown free-market ideology behind them."
Meanwhile, top doctors have said the plans for the NHS need changing so much that the entire bill may need to be withdrawn.
The British Medical Association (BMA) has called for a series of amendments as part of its submission to the listening exercise.
In particular, it is calling for the role of Monitor to be changed from enforcing competition to promising integrated staff.
And the BMA is calling for the duty of the health secretary to provide comprehensive health services to be retained.


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