Ministers have been accused of taking a "massive gamble" with the NHS after unveiling the biggest shake up in its history.
Doctors, nurses and other health workers warned plans to give GPs control of NHS budgets worth 80bn, at the same time as implementing a massive 20bn savings programme, risked damaging patient care.
A total of 24,500 administrative posts, including around 2,500 jobs in Yorkshire, will be lost as the Health and Social Care Bill scraps all 151 of England's primary care trusts alongside 10 strategic health authorities. Almost 21,000 will be through redundancy while the rest involve people leaving the service or retiring.
Family doctors will be handed control of spending decisions as other reforms signal a bigger role for private business in the NHS and in particular a controversial move to competition over prices.
The cost of implementing the changes is 1.4bn but Health Secretary Andrew Lansley insists they will save the NHS more than 5bn by 2014/15 and 1.7bn every year thereafter.
Bridlington GP Hamish Meldrum, chairman of the British Medical Association, said: "Ploughing ahead with these changes as they stand, at such speed, at a time of huge financial pressures, and when NHS staff and experts have so many concerns, is a massive gamble."
He welcomed the involvement of GPs in planning and shaping services but said other parts of the Government's proposals threatened to undermine the NHS by allowing private firms to chase the most profitable contracts.
"In particular, the legislation will allow competition to be forced on commissioners, even when they believe the best and most appropriate services can be provided by local hospitals," he said.
Peter Carter, chief executive of the Royal College of Nursing, said "the stakes could not be higher".
Staff had worked tirelessly over the past decade and patients had benefited from lower waiting times, better outcomes on cancer and cardiac care and more staff.
"It will be very important that none of the recent improvements to the NHS are placed in jeopardy as a result of these reforms," he added.
Sir Richard Thompson, president of the Royal College of Physicians, supported the shift towards putting "clinicians and patients in the driving seat". But he added: "The scale and pace of change – and the challenge of unprecedented efficiency savings – should not be under-estimated. Neither should the risks if we get this wrong."
TUC general secretary Brendan Barber branded the plans "the Government's ticking time bomb".
"The Conservatives told us before the election that there would be no cuts to the NHS, no top-down reorganisation and respect for the founding principles of the NHS. Today's plans break each of those pledges," he said.
Karen Jennings, of Unison, said: "These changes are undemocratic, they were not in any party manifesto and no one has been given the chance to vote on them."
The reforms brought fierce exchanges at Prime Minister's Questions in the Commons yesterday.
Labour leader Ed Miliband told Mr Cameron: "Doctors and nurses say your reforms are extremely risky and potentially disastrous. Why are you so arrogant to think you are right and all of the people who say you are wrong are wrong?"
Mr Cameron said the Government was "reforming the NHS so that we have got the best in Europe". He added: "The whole aim of these NHS reforms is to make sure we get the value for the money we put in."
Documents with the plans point to possible "risks", including the potential loss of key staff. They also outline how GPs will receive a "quality premium" linked to patient care and how well finances are managed to create a financial incentive.
Mr Lansley said: "Modernising the NHS is a necessity, not an option – in order to meet rising need in the future we need to make changes. We need to take steps to improve health outcomes, bringing them up to the standards of the best international healthcare systems, and to bring down the NHS money spent on drugs."