Deputy Prime Minister Nick Clegg today vowed to veto the Government's planned NHS reforms unless the package put forward by Health Secretary Andrew Lansley is improved.
Mr Clegg said unless there are "substantial, significant changes" to Mr Lansley's proposals to hand commissioning powers to GPs and extend private provision of NHS services, he will tell Liberal Democrat MPs and peers to vote them down.
The warning came as Mr Clegg set out his plans to be more assertive within the coalition Government in response to the drubbing received by the Liberal Democrats in last week's elections.
The Lib Dem leader accepted his party's traditional supporters were "anxious" about the programme of spending cuts which the Government has signed up to, but insisted that the coalition's efforts to reduce the deficit must continue.
Despite the loss of more than 700 English councillors and the bulk of his representation in the Scottish Parliament - as well as the overwhelming defeat on electoral reform - Mr Clegg insisted the Lib Dems still had "a platform from which we can recover".
His comments came as Labour leader Ed Miliband issued a plea to disgruntled Lib Dems to jump ship from the coalition and join him in opposing the Conservatives - a call which was dismissed as "tactics" by Mr Clegg.
The Lib Dem leader told BBC1's Andrew Marr Show: "I accept that there is real pressure on us to explain to our traditional voters why we are doing this and why it is good for the country.
"Of course there are lessons to be learnt, and the lesson I have learnt listening to people on the doorsteps is that people want a louder Liberal Democrat voice in Government."
And he made clear that the NHS will be the first battleground on which he will attempt to demonstrate the Lib Dems' influence.
He insisted that the "pause" in the legislative process announced by Mr Lansley will not simply amount to a cosmetic consultation with clinical staff and patients.
"It is absolutely not just a pause for the sake of it," said Mr Clegg.
"This is not a cosmetic exercise. There will be substantial, significant changes to the legislation.
"As far as Government legislation is concerned, no bill is better than a bad one, and I want to get this right. Protecting the NHS, rather than undermining it, is now my number one priority.
"I am not going to ask Liberal Democrat MPs and peers to proceed with legislation on something as precious and cherished - particularly for Liberal Democrats - as the NHS unless I personally am satisfied that what these changes do is an evolutionary change in the NHS and not a disruptive revolution."
Mr Clegg said critics of Mr Lansley's plans were "right" to warn that changes must not be pushed through too fast and GPs should not be forced to take on commissioning roles before they are ready.
And he promised: "What you will see in this legislation are clear guarantees that you are not going to have back-door privatisation of the NHS."
Mr Clegg insisted it was not the time for "tit for tat politics in the Government (with) ministers fighting like cats and dogs" and said there would be no rewriting of the coalition agreement.
But he told Andrew Marr: "I think all governments evolve and there's a natural evolution in a coalition Government.
"In the first instance, you have to work together to take lots of difficult early decisions, but of course over time, your separate identities come out more.
"The message I've heard on the doorstep is people want to hear a louder Liberal Democrat voice in Government. It's already very loud inside Government, we've got to make sure that people hear it outside Government."
Meanwhile, Ed Miliband appealed to Liberal Democrat ministers to pull out of the coalition.
The Labour leader said local election results showed voters resented the Lib Dems propping up the Conservatives.
He told the show: "Liberal Democrat ministers must hear that they had a real message from the electorate from their former voters. The message was 'we didn't vote for many of the things you're doing. You are being led by the nose by a Conservative Government. Change course on tuition fees, on economic policy."'
And he addressed Lib Dem Cabinet ministers such as Vince Cable and Chris Huhne directly, saying: "You should change course, you should have the courage of convictions you went before the electorate with a year ago.
"Frankly, if I was in your position and didn't get that change in direction, then I wouldn't stay in this Government."
Mr Miliband claimed voters saw Labour was a suitable opposition, and pointed to gains in Thanet and Thurrock, and winning control of Gravesham, as evidence of a resurgence in southern England.
He said: "We have got to be a truly national party fighting in the south as well as the north. We made progress.
"But there is a long way to go and I'm the first person to recognise we have a long way to go to recover from where we were at the last general election and win back the trust of people to win the next general election."
He added: "We need to reach further. But people think we're a credible opposition to this Government. We have got to convince more people about the alternative there is."
Chancellor George Osborne hailed Tory gains and said the coalition would continue to tackle the record budget deficit.
Mr Osborne clashed with Mr Huhne around the Cabinet table last week after the Energy Secretary apparently confronted the Chancellor over Alternative Vote campaign tactics.
"There's a lot of heat in an election campaign in any democracy. Of course things are said, but we've had the result and now we move on," Mr Osborne told today's programme.
"We fought a very good local election campaign, local Conservative councils were rewarded. Not for 30 years has a governing party increased its vote share on the last general election."
He insisted the coalition would survive the parliament, adding: "We are absolutely committed to delivering on that."
Mr Clegg set out how the Lib Dems' new approach would work in Government: "Where we achieve Liberal Democrat policies in Government, we have got to tell people about it.
"Fairer taxes, better pensions, more apprenticeships, entitlements for two-year-olds for pre-school support, a better cash settlement for schools - all of these things wouldn't have happened without Liberal Democrats in Government.
"We need to show people where we have a moderating influence on the Conservatives and we need to stand up for our values and say that loud and clear."
But he insisted the coalition did not need to "reinvent the wheel" by rewriting the agreement which sealed their partnership last year.
And he made clear he was not proposing to reverse the Government's central economic strategy: "What would be wrong is to say that somehow this Government - and it was a collective judgment - was wrong to say that we need to get the job done before the next general election of restoring prosperity and confidence to the British economy.
"That remains a crucial vocation for the Government."
Mr Clegg vowed there will "absolutely unambiguously" be further increases in the income tax threshold during this Parliament to take more of the lower-paid out of the tax altogether - a key Liberal Democrat priority.
And on the NHS, Mr Clegg promised that no GPs would be required to take on a commissioning role before they are ready.
He said: "A lot of people have said to me - and I basically think they are right - 'You are going too fast, you are trying to meet artificial deadlines, you are forcing GPs to take on commissioning roles when they might not want to and aren't able to'.
"I basically think they are right. I think what we should now do is an evolutionary approach, that this change only happens where people are willing and able to take on these new changes. If not, we shouldn't be forcing the pace according to artificial deadlines in the calendar."
Defence Secretary Liam Fox told Sky News's Murnaghan programme: "As the dust settles over the weekend and people focus back on the economy and some of the major issues facing us, they will recognise that there is still an economic emergency out there and the main reason for the mission of the coalition is still valid."
Dr Fox insisted he did not expect Mr Clegg to seek confrontation in Cabinet: "I always found Nick Clegg somebody who is extremely straightforward to deal with and personally very easy to deal with.
"I don't think his style of politics is to demand concessions. It will be to look to the coalition agreement and over time to show that there has been success in bringing the programme together."
He played down the suggestion that Mr Clegg will be looking for opportunities to defeat his Conservative coalition partners on issues like the NHS. It was entirely normal for MPs and ministers to try to improve legislation as it passes through Parliament, he said.
"It is not a question of victory for Conservatives or Liberal Democrats, it is whether we get the policies right in the longer term," said Dr Fox.
"We have had this pause in the NHS reforms to try to make sure we have got everybody on board - not just the public, not just the politicians, but the professionals. That exercise is ongoing."
Shadow education secretary Andy Burnham challenged Liberal Democrat MPs to show their independence from the Tories by voting for Labour amendments to the Education Bill on Wednesday.
Mr Burnham said the Bill as currently tabled would create a "free-for-all" in education, and his amendments would require all teachers employed in state schools to be properly qualified, ensure a fair admissions system and provide good quality careers advice for all pupils.
"These are three common-sense simple amendments to the Bill and we are looking to the Liberal Democrats to help us carry them on Wednesday," he told Sky News's Murnaghan programme.
"We are saying to the Liberal Democrats we can better reflect what the country voted for last May if you work with us to stop these right-wing reforms to our public services.
"I think that the new phase in politics we are about to enter, the test for the Liberal Democrats is - with this language about change and a new phase in the coalition - if they really mean it, they need to come with us and help us stop some of these reforms."
Mr Burnham, who is Labour's election co-ordinator, said he was not ruling out a snap general election as early as this year.
"Some of the language coming from Liberal Democrat Cabinet ministers - Vince Cable in particular - it is extraordinary for these kinds of statements to be made about colleagues in the Cabinet," he said.
"You can't help but think that there is serious trouble ahead for the coalition, so I look to the possibility that an election could come at any time and I am encouraged by the progress that Labour has made."
While accepting that Labour had a "bitterly disappointing" election to the Scottish Parliament, Mr Burnham insisted that the party had made "real progress" in England and Wales, just one year after their poor showing in the 2010 general election.
"In 1998, the Tories had nowhere near as much progress in local elections as we can report, so I am encouraged," he said.
"We did get good results in England - we topped the poll for the first time in a decade, and if that is not a sign of real progress then I don't know what is.
"Labour is coming back and we are winning in the areas we need to."