The Government today described as "disappointing" a call by the British Medical Association (BMA) for it to withdraw or significantly amend its health reforms, which posed an "unacceptably high risk to the NHS".
BMA chairman Dr Hamish Meldrum said there was an "inappropriate and misguided reliance on 'market forces"' to shape services, which could have long-term knock-on effects.
In an letter sent to every MP, Dr Meldrum said: "It is clear that the troubled passage of the Health and Social Care Bill reflects real concern over the future direction of the health service in England."
He said the BMA acknowledged government efforts to address concerns about the bill, but added: "However, we still believe that the Government's reform plans pose an unacceptably high risk to the NHS, threatening its ability to operate effectively and equitably, now and in the future.
"This is why the BMA continues to call for the Bill to be withdrawn or, at the very least, to be subject to further, significant amendment."
The letter said widening patient choice to "Any Qualified Provider" (AQP) across a larger range of services could destabilise local health economies if not carefully managed.
It also said not enough thought had been given to the "unintended knock-on effects and long-term consequences" of proposals in the bill.
The letter said focus on the changes from the reforms was creating a "noticeable distraction" from efforts to improve the quality of patient care at a time when the NHS was working to save £20 billion in efficiency savings.
It said: "The risks are high, not least because the long-term effects of the legislation are likely to be extensive.
"Meaningful, sustainable reform needs to have the full confidence of patients and those working in the health service."
In an interview with the Guardian, Dr Meldrum said the reliance on market forces could mean that hospitals would be forced to treat wealthy foreigners rather than poor patients to raise cash.
He told the newspaper the reforms would see the NHS become a market-based health system like the United States.
"There, those who pay or are insured get a better service than those who do not and rely on state-funded Medicare. Until now our system has been built on social solidarity where patients get appropriate treatment in the appropriate time."
He said trusts were being encouraged to concentrate on profitable areas of work rather than the most essential.
Last month leading union Unison warned the health reforms would take the cap off the number of private patients hospitals can treat, pushing those on the NHS to the bottom of waiting lists.
The union's head of health, Christina McAnea, said: "The public do not want a health service where people can buy their way to the top of the NHS queue.
"The end of the limit on the number of private patients hospitals can treat, will lead to less profitable NHS patients being pushed to the back of the ever-growing waiting lists."
The Government previously maintained the Bill, which will resume its passage through Parliament after the summer recess, is a crucial part of its vision to modernise the NHS so that it is "built around patients, led by health professionals and focused on delivering world-class healthcare outcomes".
A Department of Health spokesman said: "The BMA's campaign is disappointing because as the doctors' union they previously said they were pleased that the Government has accepted the Future Forum's core recommendations, and that there will be significant revisions to the Health and Social Care Bill.
"The independent NHS Future Forum confirmed the NHS must change to safeguard it for the future. They also found the principles of our plans - such as handing more power to doctors and nurses and putting patients at the heart of the health service - are well supported.
"We will never privatise the NHS and patients will never have to pay for NHS care. Our plans have been greatly strengthened in order to safeguard the future of the NHS."
Chairman of the independent NHS Future Forum Steve Field said: "Every health system in the developed world faces the challenges of rising demand, an ageing population and increasing costs of treatment. These challenges will not be met by the NHS doing more of the same. They require a culture that centres on patients and makes better value of available resources.
"It became clear during the listening exercise that the NHS had to change. So we were pleased that the Government listened and put forward over 180 amendments to the Health and Social Care Bill to improve its plans to safeguard the future of the NHS. The old hospital based system has to develop into a more preventative, community based system."