The reforms were first set out in a white paper published last summer David Cameron has strongly defended the coalition's plans for a major overhaul of the NHS in England, as MPs prepare to hold their first debate on the bill.
Writing in the Times, the prime minister said the health service was heading for crisis if it failed to modernise.
Under the plans, GPs will get control of £80bn of the NHS budget from 2013.
Health unions have denounced the changes, and Labour say the Lib Dems may be blamed for any consequences.
Health workers from around England are expected to protest against the government's bill in London on Monday.
The general secretary of the union Unison, Dave Prentis, said: "This titanic reorganisation threatens to sink the NHS. The government should step back from the brink and pronounce this bill DOA - dead on arrival."
In the newspaper article, Mr Cameron says he wants to address the myths that have emerged about the controversial plans to re-organise the NHS.
He warns that the growing number of people needing health care and rising costs of drugs mean that if the NHS fails to modernise, it is heading for crisis.
David Cameron“This is about the freedom of GPs to choose whatever is best for their patients”
"Already our health outcomes lag behind the best in Europe," he said. "Without modernisation, the principle we all hold dear - that the NHS is free to all who need it, when they need it - will become unaffordable."
The plans, published in the Health and Social Care Bill earlier this month, will hand £80bn to groups of GPS to "buy in" the bulk of hospital and community services for their patients.
Mr Cameron says the changes are "not revolution", but "evolution", building on practices that already exist and he denies the government is introducing privatisation by the back door.
"This is about the freedom of GPs to choose whatever is best for their patients," he said. "That's not privatisation; it's progress."
Shadow health secretary John Healey has warned Liberal Democrats they may be blamed for the consequences of the changes.
He has written to Lib Dem MPs, saying the changes are driven by Conservative ideology and pose a very serious risk to the NHS.
The BBC's political correspondent Carole Walker said that, despite all the controversy, the government was not anticipating a significant rebellion.
On Sunday, Health Secretary Andrew Lansley admitted there was "risk" involved in the shake-up.
"But actually if we don't change, the greater risk is that these problems we have at the moment won't be solved," he added.
The NHS Confederation has warned that hospitals could go bust by opening up the NHS to "any willing provider".
Critics have also questioned whether GPs have the experience and skills to handle such huge budgets - they will have control of about 80% of the NHS budget.
Under the plans, all 151 primary care trusts and strategic health authorities will be disbanded.
So far, 141 GP consortia, serving more than half of the population of England, have signed up as "pathfinders" to pilot the new arrangements.
The changes were first set out in a white paper published last summer. They apply solely to England - Scotland, Wales and Northern Ireland have different systems.
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Source: http://www.bbc.co.uk/go/rss/int/news/-/news/uk-12321166
