David Cameron today sought to quash speculation over the survival of Health Secretary Andrew Lansley and his controversial NHS shake-up.
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".
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.
"But modest spending increases without reform will not work," he went on. "The failings of the current set-up are too profound and the future pressures are too great. But I want to reassure people that the change we propose is evolutionary, not revolutionary.
"Because while Labour wasted money on bureaucracy and vanity projects like the NHS Super Computer, and while top-down targets distorted some clinical priorities, there was the start of sensible reform.
"Payment by results began, patients got limited rights to choose a hospital, and competition was expanded. We need to build on this - and that is what the Bill does."
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.
"Choice, competition and transparency may unsettle some people," he wrote. "But it's these things at the heart of our reform that will lead to the better NHS I care about and our country deserves."
The Government has already "paused" the Health and Social Care Bill and accepted dozens of amendments since it was first introduced.
But the concessions have failed to quell protests from professional bodies such as the British Medical Association and the Royal College of Nursing.
The legislation has already suffered one defeat since reaching the Lords and there are fears the process could drag on into next month.
Shadow health secretary Andy Burnham called on the Prime Minister to drop the Bill, adding Labour would be prepared to talk with ministers about introducing GP-led commissioning under existing legislation.
But if the Government didn't back down and drop the Bill, Labour would fight it "tooth and nail", Mr Burnham added.
He accused Mr Cameron of "putting his political pride before the best interests of the National Health Service", telling Andrew Marr the Government should now publish the details of its own impact assessment of the re-organisation.
Mr Burnham said: "It is inescapable that this is the wrong time to re-organise the NHS. The effect of doing it is putting services at risk, so we are seeing waiting lists beginning to rise around the country... we are seeing job losses around the system, we are seeing random rationing. There are signs of an NHS in increasing distress so this re-organisation is only adding to that uncertainty."
He added: "I have never argued that the NHS is perfect but the coalition inherited a successful, self-confident NHS and in just 18 months they have turned in to an organisation that is demoralised, destabilised and fearful of the future.
"When we left Government, patient satisfaction with the NHS was at an all-time high, waiting times were at all-time low. The question I would ask is why did the Government take that situation and just throw all the pieces of the jigsaw up in the air with this huge re-organisation?"
Culture Secretary Jeremy Hunt dismissed the idea of Mr Lansley being axed. "The first thing I want to say is that Andrew Lansley is absolutely the right person for this job," he told Marr.
"Andrew Lansley is a decent man, passionate about the NHS and he knows what he is doing."
Mr Hunt said Mr Lansley would be seen as the "architect of the modern NHS" in the future.
"It is completely wrong to make a judgment about someone when they are right in the middle of the storm," he added.