The government's contentious NHS reforms are an "unnecessary and unwanted" upheaval, the British Medical Association (BMA) has said.
BMA chairman of council Dr Hamish Meldrum warned ministers that the union would hold them to account "every step of the way" as the legislation rolls out across the country, the Press Association reported.
The Health and Social Care Act became law in March after a tortuous passage through Parliament.
Referring to the "monster" legislation, Dr Meldrum told the BMA's annual conference in Bournemouth: "The NHS in England is going through its biggest - and most unnecessary and unwanted - upheaval for a generation, following the passing into law of the Health and Social Care Act."
He added: "The BMA will be monitoring closely what is happening to the NHS, what is happening to services, what is happening in terms of privatisation, what is happening to commissioning and the big companies who want to take it over - and we will hold you to account every step of the way.
"We will never give up on our NHS."
He said that rather than wondering what might have been, doctors should refocus their strategy to ensure that the NHS survives.
He added: "I don't buy the charge that the passing of the Act means the beginning of the end for the NHS. That's what they said after the 1990 legislation.
"It won't be flawed, unnecessary, unwanted legislation that will end the NHS, it won't be politicians who promise one thing and deliver another, it won't even be the private companies who are circling and hoping to get a bigger slice of the action.
"The end of the NHS will only happen if there aren't enough doctors, nurses, other health professionals and, yes, patients and the public, who are willing to fight for and really support one of the greatest British achievements of the post-war period."
Dr Mark Porter, who is favourite to take over as chairman of council at the BMA when Dr Meldrum steps down on Thursday, said the legislation has "malignant intent".
He said: "The fiasco that is the Health and Social Care Act is now law.
"Described even by the person appointed to implement it as 'completely unintelligible', much of the complexity of the Bill was designed from the start to veil the malignant intent of the legislation."
Health minister Simon Burns said: "The Health and Social Care Act will deliver more power to clinicians, it will put patients at the heart of the NHS, and it will reduce the costs of bureaucracy.
"The independent NHS Future Forum found broad support for the principles of our reforms and earlier this year, 75 GP leaders, representing thousands of patients, outlined their support for shifting power to the front line.
"Without our reforms, doctors and nurses would have continued to run the risk of having their decisions second-guessed by the managers running primary care trusts.
"The Act cuts out this needless bureaucracy, reinvests £4.5 billion in patient care, and hands control for care over to those who know their patients best - the doctors and nurses throughout the NHS."