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Ministers have hit back at critics as they unveiled legislation seeking to shake-up the NHS in England, saying change is a "necessity not an option".

The Health and Social Care Bill will allow GPs to get control of most of the NHS budget from 2013.

Ahead of its publication, unions had warned it could undermine the NHS.

But Health Secretary Andrew Lansley spelt out how essential services would be protected and GPs supported, saying it was not as radical as claimed.

Speaking after the bill was laid in Parliament, Mr Lansley accepted there were "new and distinctive" elements but said it was more about evolution as it built on many of the changes introduced by Labour.

Choice, use of the private sector and setting hospitals free as foundation trusts were all started under the previous government.

He said unions were always "against competition", but they were wrong in saying it would lead to the destruction of the health service.

He said: "Modernising the NHS is a necessity, not an option, in order to meet rising need in the future we need to make changes."

Under the plans, GPs working in consortia will take on responsibility for "buying in" the bulk of hospital and community services for their patients.

In the process, all 151 primary care trusts (PCTS) and strategic health authorities will be disbanded.

In the lead up to the bill's publication, fears were voiced that hospitals could go bust as the plans include 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.

The timing has also been questioned, as the health service is being asked to save money in coming years.

But in setting out the details of the bill, Mr Lansley said the national independent board would support GP consortia as they were established.

He also said while trusts could go under regulators would be able to step in to ensure "essential" services continued.

And documents released alongside the bill claimed the changes would save money.

While the overhaul will cost £1.4bn, this would be paid for through savings within two years, the paper suggested.

By 2015, £5bn could be saved through cutting the number of people working in management and administration.

Earlier, during exchanges at prime minister's questions, David Cameron was accused of being "arrogant" for going ahead with the moves despite warnings from unions and health experts.

Labour leader Ed Miliband said medical staff had warned of "potentially disastrous" consequences for the health service.

But the prime minister said the government was "reforming the NHS so that we have got the best in Europe".

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.

But despite Mr Lansley's attempts to answer the critics, health staff and independent experts still said they were worried.

Karen Jennings, head of health at Unison, said: "This Titanic health bill threatens to sink our NHS. The only survivors will be the private health companies that are circling like sharks, waiting to move in and make a killing."

Professor Chris Ham, chief executive of the King's Fund health think-tank, was more measured.

He said getting GPs more involved was a good move, but warned the changes were at risk from "the combination of the funding squeeze and the speed and scale of the reforms".

Shadow health secretary John Healey said: "It's a huge upheaval which will put unnecessary extra pressure on the NHS and could open up all parts of the NHS to competition from private health companies."