Ed Miliband called for a concerted effort to force the Government to abandon its health reforms, warning critics had only three months to halt a "dangerous leap in the dark" for the NHS.
The Labour leader sought to galvanise opposition to the Health and Social Care Bill which faces a fresh hurdle next week when it returns to the House of Lords for further debate.
Despite tabling a slew of amendments to the controversial legislation, Health Secretary Andrew Lansley is still battling in the face of opposition from many parts of the health profession.
Mr Miliband said the shake-up would open the health service to a "free market free-for-all" that would put the principles of the NHS at risk and insisted the Government could still drop the plan.
Writing in The Observer he said: "True, words like co-operation and integration have been inserted into the legislation, but the Government has not backed down from the original concept.
"The reality is that this still represents a dangerous leap in the dark, putting the principles of the NHS at risk."
He dismissed claims by the Prime Minister that the opposition came from "vested interests" in trade unions, insisting hundreds of thousands of doctors, nurses, midwives and others were angry.
"David Cameron has always said he wants to make GPs' voices stronger in the NHS. So why doesn't he start by listening to them now?," he asked.
"People at the heart of the NHS, staff and patients, would breathe a sigh of relief if the Bill was dropped. Doctors and nurses could get back to their real job - of patient care.
"At the last election, David Cameron cited his commitment to the NHS to show he was a different type of Conservative. And he promised no more top-down reorganisations.
"But all he has done is betray his promises and let people down.
"It is not too late to stop this Bill. We have three months to prevent great harm being done to the NHS. Now is the time for people of all parties and of none, the professions, the patients and now peers in the House of Lords to work together to try to stop this Bill."
Mr Miliband rejected the PM's comparison of himself with Labour predecessor Tony Blair who said of unpopular reforms that opposition was "inevitable but it is rarely unbeatable".
"A sign of a reform being on the right track is whether it slowly builds support, as the Labour government did with the tough action we took to get waiting times down, including using the private sector where appropriate," Mr Miliband wrote.
"Cameron's reforms have done the opposite as every week a new group of professionals or patients turns against his plan. He has squandered any trust he once might have had from people on the issue of the NHS."
Mr Lansley insisted he had support from doctors and nurses for the "core principles" of the Bill and pointed the finger at trade unions.
"Our plans to improve the NHS are essential if it is to be sustainable for the future," he told the newspaper.
"Doctors and nurses across the country support the core principles behind the reforms - giving more power to clinicians to design services for patients, getting patients the information they need to make proper choices and promoting democratic accountability, with councils leading health improvement.
"Any reform of something as important as the NHS will cause controversy.
"Trade unions like the BMA opposed the very creation of the NHS. Labour used to support reform but now they are jumping on the bandwagon of opposition in order to please their trade union masters."