One of Nick Clegg's closest advisers has threatened to quit unless ministers make changes to a proposed overhaul of the NHS.
Lib Dem MP Norman Lamb said the plans posed a major "financial risk" to the NHS, and patient care could suffer.
He said he would quit as Mr Clegg's chief political adviser unless NHS professionals were "on board".
Health Minister Simon Burns said while there was consensus over the principles of the plan, it could be improved.
Labour said Mr Lamb's comments "added to the confusion" the government was showing over the NHS.
The government is consulting further on plans to overhaul the NHS, under which GPs are to be given control of much of the budget, tiers of management axed and the service opened up to more competition.
The "listening exercise" was launched following widespread criticism of the plans by opposition parties and medical professionals, including the British Medical Association and the Royal College of Nursing (RCN).
Lib Dem activists have also aired their concerns, voting against the restructuring at their Spring Conference.
Mr Lamb, the MP for North Norfolk who was the party's health spokesman in opposition, said the planned changes were an "enormous political risk".
He told the BBC's Politics Show that the proposal to give GPs control of budgets should be phased in rather than "rushed into".
"The sensible thing to do is to test it and see how it works," he said. "To do it in one fell swoop is very risky".
While supporting the general direction of government proposals, he feared there was "no evidence" how the new GP-led system would operate.
"My real concern is the financial risk of doing it too quickly, because then services and patient care suffers," he said."
Mr Lamb said he felt "very strongly" about the issue - so much so that he was prepared to stand down if aspects of the changes were not modified.
"I've said that if it's impossible for me to carry on in my position, I will step down. And I think that it's in the government's interest to get it right in the way that I suggest.
"Getting the NHS right is the most important thing. And indeed it would be incredibly destabilising politically if we get this reform wrong."
Mr Lamb stressed he was speaking in a personal capacity and had told the deputy prime minister of his intention to voice his concerns.
Also speaking on the BBC's Politics Show, Conservative Party deputy chairman Michael Fallon acknowledged the government "needed to get the reforms right, which means listening and seeing whether we can improve what we've already produced".
And Conservative health minister Simon Burns said the government was committed both to the principles of NHS and modernising the service.
"Our plans reflect both Lib Dem and Conservative thinking and implementation on the ground continues to be impressive," he said.
"While there is broad consensus over the principles of our plans, we are taking the natural break before it moves to the House of Lords to pause, listen and reflect so we can make more improvements."
However, shadow chancellor Ed Balls said he thought Mr Lamb's comments were "very significant".
"What we don't want is a pause or a PR initiative, what we want is for the government to say we've got this wrong, and as Norman Lamb was saying, have a change of direction," he said.
And shadow health spokesman John Healey said Mr Lamb's comments "added to the confusion throughout the Conservative-led government over its handling of the health service" and showed the plans were "flawed".
Also speaking on Sunday, Treasury Chief Secretary Danny Alexander acknowledged there were "issues" in the way GP-led commissioning consortia would operate and be regulated.
"We intend to come back with serious, substantive changes to this Bill as a consequence of this process," he told the BBC's Andrew Marr programme.
Peter Carter, general secretary of the Royal College of Nursing, said ministers should listen to Mr Lamb as he was "someone who knows the NHS".
As well as being concerned about the pace of change, he suggested the government should seek to broaden the range of experts involved in the GP consortia as recommended by MPs on the health select committee.
He said the changes would be "high risk" at any time but added: "To do it now when at the same time stripping out £20bn is a recipe for real instability and it will be patients that will suffer."