Attempts by Liberal Democrat peers to re-write key elements of Andrew Lansley's health reforms have the support of Nick Clegg, it has been claimed.
Ministers are braced for further controversy over the government's Health and Social Care Bill when it resumes its troubled passage through the House of Lords on Monday.
Lib Dem peers are seeking to push through a series of detailed amendments- including measures to strip out proposals to increase competition in the NHS.
David Cameron has made clear that he intends to see the legislation through, warning last week of "chaos" in the NHS if the reform proposals were abandoned now.
And George Osborne said at the weekend that it was essential the changes went through with the NHS was to be affordable in future while meeting the needs of an ageing population.
But with Lib Dem activists threatening a revolt at the party's spring conference next month, Labour sought to keep up the pressure - urging Lib Dem peers to vote with them to kill off the bill altogether.
While that looks unlikely, Labour leader Ed Miliband said that it was still not too late for Cameron to think again and abandon the legislation.
"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."
Baroness Jolly, the Liberal Democrat co-chair for health, said there were areas of the bill which are "seriously flawed".
The peer told BBC Radio 5 Live's Stephen Nolan programme: "What we want is an NHS which is totally sustainable, which is free for everyone as and when they need it."
She added that Lib Dems were in favour of "huge chunks" of the bill, but there were "certain elements" party members wanted to change.
"We are trying really hard to say no more competition and no more favours for the private sector and if they come out we will be delighted," she said.
Lady Jolly said later that Lib Dem peers' actions in the House of Lords had been cleared with the office of deputy prime minister.
"The party are totally supportive of what we are doing in the Lords. We have discussed this with the advisers to Nick Clegg, so his office is on board with that," she told BBC Radio 4's World At One.
At the same time, the Royal College of Physicians (RCP) - representing hospital doctors - is meeting today in emergency session to discuss whether to formally ballot the views of its 26,000 members on the legislation.
An online poll conducted by doctors opposed to the reforms found 92.5% of RCP members believed they should now be abandoned, according to The Observer which was given access to the findings.
A spokeswoman for the college said: "The RCP continues to have serious concerns about the reforms and has been lobbying vigorously for changes to the Bill since its publication."
If the RCP were to come out against the legislation, it would bring it into line with the other royal colleges in calling for the bill to be withdrawn, leaving ministers looking even more isolated.
In a further blow, former NHS chief executive Lord Crisp yesterday denounced the reform plans as "a mess", saying they were "unnecessary, confused and confusing".