David Cameron has sought to quash speculation over the survival of health secretary Andrew Lansley and his controversial NHS shake-up.
The Prime Minister insisted he was "at one" with Mr Lansley and the legislation going through parliament.
He also attacked Labour for "opportunism" on the issue - claiming the proposals were an "evolution" from changes introduced by the previous government.
The intervention comes amid growing pressure for the Health and Social Care Bill to be scrapped. Several Conservative Cabinet ministers are said to have privately condemned Mr Lansley's handling of the package, with one suggesting the problems were now on the scale of the Poll Tax in the 1980s.
A Downing Street source was also quoted last week saying that the health secretary should be "taken out and shot".
However, writing in The Sunday Times, Mr Cameron stressed that there was no alternative to reform.
The Prime Minister - whose disabled son Ivan died in 2009 - said: "As a parent, night after night, I've known what it is to have the NHS by your side. I've seen the dedication - the reassurance that if the worst happens, the NHS will be there for your family.
"That's why I so strongly support the founding principle of the NHS: health care for all, free at the point of use, unrelated to the ability to pay. That won't change. But while the values are right, the system isn't. It needs to change - and that is why I am at one with Andrew Lansley, the reform programme and the legislation going through Parliament. The shortcomings of the status quo are well known. There's too much bureaucracy - and too much decision-making is led by that bureaucracy rather than clinicians."
Mr Cameron said the Government was providing an extra £12.5 billion in this parliament to eradicate health inequalities and cope with cost pressures.
Mr Cameron insisted the Bill gave "power to doctors and nurses", and would lead to "more choice for patients and competition for treatment". The "staggering" £4.5 billion savings would be ploughed back into patient care, he added.
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