The overhaul of the NHS in England will cause irreparable harm, according to leading public health doctors.
In a letter to peers, who will debate the changes next week, nearly 400 public health experts said the changes must be rejected as they represented a risk to patient care and safety.
The doctors suggested it would fragment services, possibly threatening vaccination and screening campaigns.
But the government insisted the reforms would give patients more choice.
The revelation that such influential members of the public health community have put their names to the letter comes on the day Health Secretary Andrew Lansley is to address the Conservative Party conference.
The NHS reforms have been one of the most controversial areas of government policy over the past year.
The proposals even had to be put on hold in the spring amid mounting criticisms from the medical profession, academics and MPs.
It led to ministers making a number of concessions. These included giving health professionals other than GPs more power over how NHS funds are spent as well as watering down the role of competition.
But the intervention by the public health experts illustrates there is still widespread opposition - despite claims by the prime minister last month that the NHS now backed the changes.
Signatories include Sir Michael Marmot, one of the world's leading experts on health inequalities, and 26 directors of public health.
The letter, which has been sent to members of the House of Lords, states: "The bill will do irreparable harm to the NHS, to individual patients and to society as a whole."
It goes to say that the increased commercialisation and marketisation would "fragment patient care, aggravate risks to individual patient safety... and undermine the ability of the health system to respond effectively and efficiently to communicable disease outbreaks and other public health emergencies".
It adds: "The government claims that the reforms have the backing of the health professionals. They do not. Neither do they have the general support of the public."
Inner north-west London public health chief Dr David McCoy, who has signed the letter, said the impact could be felt in vital areas of public health such as "vaccination, TB control and cancer screening".
"Healthcare requires a co-ordinated and collaborative approach," he added.
The criticisms follow on from warnings from other groups, such as the Royal College of Nursing and British Medical Association, about the damage the changes may cause.
BMA chairman Dr Hamish Meldrum said: "Ideally, we'd like to see the legislation withdrawn entirely. Failing that, it needs to be significantly amended."
'Root out waste'
David Cameron said the letter praised the part of the bill that concerns public health.
"We're saying public health - smoking cessation, diet, dealing with things like diabetes - these are incredibly important and they welcome that part of the bill," he said.
"Of course though, there are doctors in the health service who don't like the idea of greater choice and competition and other organisations being able to provide free health services to patients.
"But I believe patients want that sort of choice and rapid quality treatment and that's why it's right to make these reforms."
Shadow health secretary John Healey said: "David Cameron is in denial, both about the damage his plans are doing to the NHS and the strength of opposition."
A Department of Health spokesman said: "Our plans to modernise the NHS will give patients more choice, root out waste and give trusted NHS staff more power to improve care.
"A group of the country's top doctors and NHS experts have already analysed the Health Bill and we have accepted all their recommendations to improve it. Many GPs and other doctors have also already spoken out in support."