Jeremy Hunt is to take over from Andrew Lansley as health secretary.
Mr Lansley, whose NHS reforms prompted months of controversy last year, will become leader of the House of Commons.
My Hunt moves from the post of culture secretary, where he survived a Leveson Inquiry examination into his relationship with James Murdoch in light of News Corporation's bid for BSkyB in 2011.
The news has been described as "disastrous" by some.
Deputy chairman of the British Medical Association Dr Kailash Chand said on Twitter "disaster in NHS carries on".
The BMA's official statement welcomed a "fresh opportunity" for doctors and government to work together to improve patient care and deal with the many challenges facing the NHS.
Mr Hunt called his new job was "the biggest privilege of my life".
Speaking outside Downing Street he said: "I'm incredibly honoured and looking forward to getting on with the job."
He will now oversee the new NHS changes and efficiency savings.
Proposals in the Health and Social Care Bill for England include giving GPs control of much of the NHS budget and opening up the health service to greater competition from the private and voluntary sector from April 2013.
But many were opposed to Mr Lansley's plans for the health service and some doctors and senior officials called for his resignation.
Professional bodies, including the British Medical Association, the Royal College of Nursing and the Royal College of Midwives, asked for the bill to be scrapped.
But the Prime Minister David Cameron said he was "at one" with Mr Lansley over the government's Health and Social Care Bill.
The bill gained Royal Assent in March 2012, more than 14 months after first being tabled in the House of Commons.
The BBC's Health Correspondent Branwen Jeffreys said Mr Hunt will have a steep learning curve, and little time to establish his credibility in one of the cabinets most taxing roles.
"He'll face scepticism and battle fatigue. The changes underway in the NHS in England are as yet largely invisible to the patient, but they touch every part of the health service. A new NHS Commissioning Board formally begins work next month, and by April large parts of the budget will be managed by GP led groups.
"Mr Hunt is swapping into the driving seat part way through the journey, and to some extent will have to keep following the road map set out by his predecessor.
"He's got just a couple of months to gather his thoughts before the first big challenge, the publication of the public inquiry report into the terrible failings of care at Mid Staffs hospital in a couple of months.
"It is likely to be the first hot potato of many he will face in his new job."