Doctors have called on the government to put a halt to its overhaul of the NHS in England, but have stopped short of opposing the plans outright.
Instead, an emergency meeting of the British Medical Association urged ministers to withdraw the health bill so the plans could be looked at again.
The plans pave the way for increased competition in the NHS - something the union said was "dangerous and risky".
But the government said the BMA should be trying to be more constructive.
The emergency meeting - the first of its kind for nearly 20 years - was called by the BMA leadership after concerns from its grassroots membership about the plans.
The motion voted in favour of by delegates says withdrawing the health bill, which is currently going through parliament, was needed so that the controversial and more radical elements of the legislation can be considered again.
But a motion calling for outright opposition of the plans was narrowly rejected, while delegates also voted against a motion of no confidence in Health Secretary Andrew Lansley.
The union fears increased competition from the private sector could harm hospitals, perhaps even forcing some to close.
BMA leader Dr Hamish Meldrum said he would rather see the NHS as the preferred provider instead of having a level playing field with the big health companies.
He said it was now time to be "ratcheting up" the concerns, but added a total opposition of the bill would send the "wrong message".
"The government's proposed reforms have far-reaching and potentially irreversible consequences for how the NHS is run and the way we deliver care to our patients."
More than 300 doctors attended the meeting with many agreeing with Dr Meldrum's concerns.
Dr Layla Jader, a public health physician, said: "The NHS needs evolution not revolution - these reforms are very threatening to the future of the NHS. If they go through, our children will look back and say how could you allow this to happen?"
And Dr Barry Miller, an anaesthetist from Bolton, added: "The potential to do phenomenal damage is profound. I haven't seen any evidence these proposals will improve healthcare in the long-term."
The meeting comes after Liberal Democrat delegates rejected the plans at the party's spring conference last weekend.
Members voted not to support a "damaging and unjustified" shake-up, which will see GPs get control of much of the NHS budget along with the scrapping of primary care trusts and strategic health authorities.
But a Department of Health spokesman said: "We are disappointed the BMA has decided to take this step rather than work constructively with us to improve services for patients.
"The reality is over 5,000 GP practices, covering two thirds of the country, have already signed up and have started to implement plans to give patients better care.
"The NHS must modernise in order to keep up with the increasing demand on services, an ageing population and rising costs of new drugs and treatments and we are passionate about ensuring high quality, efficient services for patients."
The debate comes on the day that a cross-party group of MPs set out the challenge facing the health service in making the savings it needs to keep pace with rising demands.
The Public Accounts Committee warned the target of finding £20bn of efficiencies by 2014 - about a 4% saving each year - could be missed unless the government reversed the falling productivity rates in the health service.
The MPs' report highlighted figures showing productivity had fallen by an average of 0.2% a year for the past decade, although they accepted performance was getting better thanks to increased investment in staff, equipment and buildings.
Committee chairman Margaret Hodge said: "The quality of the health service has improved as a result of this increase in spending. But the taxpayer has been getting less for each pound spent."