The reforms apply only to the NHS in England The Royal College of GPs has called for the health bill to be scrapped, heaping more pressure on the government's controversial reforms in England.
In doing so, it becomes the first of the medical royal colleges to move to outright opposition.
The college, which represents 34,000 GPs in England, said the overhaul had to be stopped because it threatened to cause "irreparable damage" to care.
A health minister said he was "baffled" by the RCGP's criticisms.
The intervention by the RCGP comes despite further amendments being made to the bill on Wednesday.
The changes included making it clear the health secretary would retain ultimate control over the NHS, and strengthening the requirement of the regulator to ensure the different providers competing for patients also work together to provide seamless services for patients.
They were put forward in response to concerns raised by the House of Lords and will be debated next week.
'No alternative'
But the RCGP said they did not go far enough and it was now time for the whole bill to be withdrawn.
It follows a similar call by the British Medical Association, Royal College of Nursing and Royal College of Midwives.
But all three of these are unions, which prompted the government to suggest they were motivated by the dispute over pay and pensions - whereas the RCGP is part of the professional arm of the health service which sets standards.
It is also significant because GPs are widely thought of as one of the main beneficiaries of the reforms, as they are supposed to get more control over how NHS funds are spent.
But the college said they could no longer work with government on the bill because its concerns over competition and bureaucracy had not been addressed.
It also said the overhaul would hamper the ability of the health service to make the savings it has been asked to in the coming years to cope with factors such as the ageing population and rising cost of new drugs.
RCGP chairman Dr Clare Gerada said: "The decision was not taken lightly, but it is clear that the college has been left with no alternative.
"We cannot sit back. Instead, we must once again raise our concerns in the hope that the prime minister will halt this damaging, unnecessary and expensive reorganisation which, in our view, risks leaving the poorest and most vulnerable in society to bear the brunt."
She told the BBC Radio 4 Today programme that the bill would "turn the National Health Service into thousands of different health services, all competing for the same patients, the same knee, the same brain, the same heart.
"Patients will find their care will be fragmented, it will be on different sites, it won't join up, it will be difficult to hand over care and it will be phenomenally expensive to keep track of all these competing parts of the NHS."
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NHS reforms timeline
- July 2010 - White Paper published, setting out plans which involve giving GPs control of the budget while encouraging greater competition
- January 2011 - Bill underpinning the changes gets its first readings in the House of Commons
- March 2011 - Lib Dem spring conference votes for radical change to "damaging" plans amid mounting concerns among health groups
- April 2011 - Bill paused so that government can carry out a two-month "listening exercise" to address criticisms
- June/July 2011 - Listening exercise leads to a series of changes to the bill, limiting competition and improving accountability
- September 2011 - Bill makes it to the Lords, leading to more amendments being tabled; expected to return to the Commons in spring 2012
- October 2011 - National board established in shadow form to oversee new system as groups of GPs start piloting new arrangements
- April 2013 - Board and GP groups to formally take control of the system
Like many of the medical royal colleges, the RCGP has to date been critical of the plans.
But it had until now been saying it was willing to work with ministers to improve them.
The entire medical royal college community has discussed coming out en masse to oppose the plans, but a draft statement that was drawn up last week was never signed off.
The BBC understands a number of other colleges are currently discussing whether to toughen their opposition.
Health Minister Simon Burns told the BBC that the RCGP had previously supported at least some elements of the bill.
But he added: "Now we have this constant criticism and I find it in one way baffling, because it's not representative of what I hear GPs up and down the country saying, now that they're beginning to engage in having the powers to determine the care that needs to be commissioned for their patients."
He added: "We have been carefully listening to the ideas raised as the bill has progressed through Parliament and, as a result, we tabled a series of amendments to safeguard the future of the NHS."
The Health Secretary Andrew Lansley is also facing a hearing in March where the Information Commissioner will seek to force Mr Lansley to release a secret government report on the risks of the NHS reforms.
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