NHS reforms could be delayed by weeks if not months, the Deputy Prime Minister has suggested.
Nick Clegg said he believed the Health and Social Care Bill would need to go back to a committee of MPs for further scrutiny.
The Bill passed the Commons committee stage at the end of March after two months of discussion before being stopped in its tracks as part of the Government's "listening exercise".
Mr Clegg said it was vital that the Government got the Bill right and insisted there would be "no arbitrary deadline" for reform.
"I think it's very important that MPs who are accountable to millions of patients up and down the country really have the opportunity to look at the details of what we are proposing," he said.
"I think that's why we will need to send the Bill back to committee."
Speaking to senior representatives from health charities and patient groups at University College Hospital in London, Mr Clegg said he did not accept the Bill was going to end up so watered down it should be abandoned.
And he said the Government was "promising evolution not revolution".
People did not want "any Government to turn the NHS upside down and inside out".
Many Liberal Democrats are hostile to what they see as moves towards privatisation of the health service - including a new duty for regulator Monitor to promote competition in the health service.
Mr Clegg told his MPs last week that making Monitor an economic regulator was a "misjudgment" and that it should be tasked with securing NHS collaboration not competition.
Mr Clegg said he backed the use of private companies in the health service and that they had improved patient choice.
But he said: "It's not the same as turning this treasured public service into a competition-driven, dog-eat-dog market where the NHS is flogged off to the highest bidder."
Mr Clegg needs to demonstrate the "muscular" influence on the coalition he promised after suffering heavy election losses on May 5.
Earlier this week, Health Secretary Andrew Lansley insisted competition between health care providers would stay a central part of the Bill.
He told The Sun newspaper: "If you have no competition you have no patient choice, but if you have patient choice you have to have rules.
"The point is for it to be done within health service values and interests."
He added: "The Bill does not change the scope of competition, it simply gives that responsibility to Monitor.
"The bottom line is, how do we deliver the best possible services for patients?
"The answer is we have to have strong commissioners who have the power to exercise their choices, who are able to design services around the needs of patients.
"If you have choice between providers then you have a degree of competition."
Consultation on "significant" changes to the controversial Tory-led shake-up of the health service is due to end next month.
Revised plans are due to be published within weeks.
Mr Clegg said it was an "unusual" decision by the Government to pause the progress of the legislation.
He said details of the changes "will have to wait" until the process is complete.
"However I can tell you that the right kind of reform starts from the patients' point of view. Not bureaucrats, not unions, not ministers, not political parties - patients."
He added: "So what do people want from their NHS? From everything I've heard over recent weeks, I would say three big things: peace of mind, the best care and a say in the decisions that affect them and their families.
"Those are the tests by which every element of the Government's package will now be judged."
There was no question of the NHS no longer being free at the point of delivery, he will say.
"No bills, no credit cards, no worries about money when you're worrying about your health. That's why I have been absolutely clear: there will be no privatisation of the NHS.
"The NHS has always benefited from a mix of providers - from the private sector, charities and social enterprises. And that should continue.
"People want choice: over their GP, where to give birth, which hospital to use. But providing that choice isn't the same as allowing private companies to cherry-pick NHS services."
Labour tabled a motion calling for the Bill to be sent back to committee.
Shadow health secretary, John Healey, said: "I welcome Nick Clegg backing Labour's motion to send the Health Bill back to the House of Commons to re-run its committee stage.
"The Government's plans for the NHS need to be radically rethought. If fundamental changes are going to be made to the legislation, they need full and proper scrutiny in Parliament."
Mr Healey used a speech to the Royal Society of Medicine to raise fears the changes will be a "political fix" and not address the threats to the NHS itself.
"David Cameron is a PR man looking for a PR answer. He must accept the problem is not the presentation of his NHS plans but the full-blown free-market ideology behind them," he will say.
"This Tory ideology is totally at odds with the ethos of the NHS and the essential way it works."
Unite, which has 100,000 members in the health service, said the Bill was flawed and heralds "commercial involvement on a scale" not seen before.
In its submission to the listening exercise, the union said it believes "the heart of the Government's proposals will transform and privatise the NHS so that services are geared towards fulfilling financial and business contractual relationships and outcomes, rather than meeting health needs".
Unison's head of health, Christina McAnea, said: "Nick Clegg's attempt to reassure people that any changes to the NHS will be in the best interests of patients, has not worked.
"A growing number of professionals are calling for the Health and Social Care Bill to be ditched and that, we believe, is the best option.
"He is completely naive to think that more competition won't lead to a greater role for private companies - companies who are chomping at the bit to start raking in cash from the health service.
"Once they get their teeth into the NHS they will destroy it."
A Department of Health spokesman said: "We are currently listening on our plans to strengthen the NHS and awaiting the report from the NHS Future Forum."
No decision on recommitting the Bill - or specific amendments to it - will be made until after ministers have responded to the NHS Future Forum report.
Mike Farrar, chief executive of the NHS Confederation, said: "We have to get these reforms right and we need to ensure they have a proper political mandate.
"But the Government must also have an eye to how long the legislative process will take.
"The NHS is under enormous pressure and we need the clarity quickly in order to make it easier for the NHS to deliver financial stability and high quality services for patients."
During a question-and-answer session following his speech, Mr Clegg insisted it would not be right to "bounce" the Bill through Parliament.
He said: "I don't think it would be right for us to hold this listening exercise - to make big changes to the legislation - and then to seek to bounce it through Parliament.
"I think it's very important that MPs who are accountable to millions of patients up and down the country really have the opportunity to look at the details of what we are proposing.
"I think that's why we will need to send the Bill back to committee.
"I have always said that it is best to take our time to get it right rather than move too fast and risk getting the details wrong.
"We will introduce substantive, big changes. My desire - I think everyone's desire - is just to get it right.
"The NHS is simply too precious, too important to millions of people in this country to rush things and get it wrong."
Shadow health secretary John Healey said: "The differences between Nick Clegg and Andrew Lansley over the future of the Health Bill confirm this is a divided, not coalition, Government.
"But those divisions are also adding more confusion and uncertainty for NHS staff and patients waiting for David Cameron to decide what changes he will make to his NHS plans."