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 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."