Nick Clegg has written to Liberal Democrat peers and MPs calling for further changes to be made to the NHS reforms.
The Lib Dem leader co-wrote the note with peer Baroness Williams who has been outspoken in her opposition to some parts of Health Secretary Andrew Lansley's bill.
He praises the work members have done to improve the Health and Social Care bill but also sets out what he believes should be altered.
Issues include reducing the amount of competition allowed in the health service and ensuring the independent regulator Monitor puts patients first.
Mr Clegg also writes that he wants to "rule out beyond doubt any threat of a US-style market in the NHS".
He emphasises the NHS should not be treated like the privatised gas, electricity or water industries.
"Once these final changes have been agreed, we believe conference can be reassured that it has finished the job it started last March and the Bill should be allowed to proceed," the letter adds.
His intervention comes as Lib Dem peers seek further adjustments to the Bill in the Lords.
But the prospect of further government concessions may anger backbench Conservative MPs.
A source close to the Lib Dem leader said Mr Clegg remains in favour of reforming the NHS.
"Shirley and colleagues in the Lords and Commons have been absolutely instrumental in the changes to the Bill so far and she and Nick have written to their parliamentary colleagues to set out the party's position ahead of the debate in the House of Lords," the sources said.
They added: "We've always said there will be no privatisation of the NHS on our watch."
Mr Clegg's letter could be an indication the Government is prepared to compromise to get the Bill through Parliament after more than 1,000 amendments were added.
The coalition's Health and Social Care Bill will give GPs more power to commission services, increase the role of private and third sector providers and see Primary Care Trusts scrapped.
But there has been opposition to the overhaul in Westminster, among medical groups and from voters.
As the Lords prepared to meet, a small group of protesters from Keep The NHS Public held a demonstration in the roads outside Parliament.
The Royal College of Physicians (RCP) , which represents hospital doctors, has chosen to ballot its 26,000 members on the legislation.
It follows an online poll - held by doctors who oppose the plans - which suggested 92.5% of RCP members thought the Government proposals should be abandoned.
"The RCP continues to have serious concerns about the reforms and has been lobbying vigorously for changes to the Bill since its publication," a spokeswoman for the organisation said.
Other groups have already said the Bill should be scrapped, including the Royal College of GPs which warned of "irreparable damage" to the health service.
The latest professional association to speak out against the reforms is the British Geriatrics Society (BGS) .
They warn older people are put at risk by fragmented services and adds there is the "serious risk of undermining progress made in recent years".
Speaking ahead of the debate in the Lords, Liberal Democrat peer Baroness Jolly said parts of the Bill were "seriously flawed".
Party members support "huge chunks", she told the BBC, but are working "really hard to say no more competition and no more favours for the private sector".
Labour leader Ed Miliband has repeated calls for the Bill to be abandoned entirely.
"Throwing all the pieces of our NHS up in the air and seeing where they land is not the right way to go about reform," he wrote in an article in The Times.
"Only political pride is preventing this Prime Minister from dropping his bill. If he ploughs on, he will not only destroy trust in himself, he will also prevent the real change that the NHS needs."
But David Cameron has indicated he plans to persevere, warning of "chaos" in the NHS if the proposals are not implemented in full.
The Government believes there need to be changes if the health service is to be sustainable, particularly as people are living longer.
In addition, several changes have already begun with some GPs joining commissioning consortia and some Primary Care Trusts being dismantled.