David Cameron on Monday embarked on his fight to "get a better deal for the British people" in Europe to resounding cheers from 330 newly elected Tory MPs, although there were signs that party unity may not last.
Mr Cameron was greeted with a thunderous banging of desks in a House of Commons meeting room as he vowed to "renew" Britain's relationship with Europe, repeating his promise of a referendum on the country's EU membership by the end of 2017.
There were declarations of loyalty from some of the same Tory MPs who had harried Mr Cameron's predecessor John Major, as he struggled to ratify the EU's Maastricht treaty in the early 1990s.
Bill Cash, a leading eurosceptic, said at the packed meeting: "Anyone who thinks we are going to disrupt things, can put a sock in it." Meanwhile John Redwood, another Maastricht rebel, promised to stand behind Mr Cameron.
"You can read that two ways," muttered one minister loyal to the prime minister. "It depends what John has in his hand at the time." Another Tory MP said: "We've written down what they've said."
Eurosceptics argue that things are different to the Major years because they have the option to vote for Brexit in a referendum if they do not like the package that the prime minister has negotiated.
David Davis, another former Maastricht rebel, has estimated that as many as 60 of Mr Cameron's MPs will campaign for Britain to leave under almost any circumstance.
Although Mr Cameron believes such a split can be managed during the course of a referendum campaign, Tory eurosceptics are already ratcheting up their demands on the prime minister.
Mr Davis suggested at the weekend that Mr Cameron should negotiate an "opt-out" for Britain over any EU legislation it did not like - a demand likely to be rejected by other member states.
As one Tory minister put it: "The French would like that idea: they could opt out of every piece of single market legislation."
By raising the stakes ahead of the renegotiation, hardline eurosceptics are setting the bar so high for Mr Cameron that some will almost certainly label his efforts a failure.
The eurosceptics are also raising the stakes in another area: they want Mr Cameron to allow Conservative ministers a free vote in the referendum, so that they could campaign for a Brexit.
Mr Cameron, who says he wants Britain to stay in a reformed Europe, made clear in January that while Tory backbench MPs could vote for Britain to leave the EU, he expects ministers to toe the government line.
Graham Brady, chairman of the backbench 1922 committee, told Radio 4's Westminster Hour that cabinet ministers should be given a free vote in order to avoid a "rancorous split" within the party.
Some cabinet ministers have already signalled that they could contemplate Britain leaving the EU, including Philip Hammond, foreign secretary, Michael Gove, justice secretary, and Chris Grayling, leader of the Commons.
"It's going to be difficult for them," said one eurosceptic grandee. "They have already made it clear where their hearts lie."
Harold Wilson, the former Labour prime minister, did allow his cabinet to argue on both sides of the 1975 referendum on whether Britain should remain part of what was then the European Economic Community.
Speaking ahead of his meeting with the new cohort of Tory MPs, Mr Cameron admitted the renegotiation would be "tough" but said he had a mandate and had already begun talks with some European leaders.
Chancelleries around Europe want Mr Cameron to be more explicit about his demands, but the prime minister's allies say that outline of his strategy is already in the public domain in a series of speeches and newspaper articles.
They include curbs on welfare for migrant workers, more powers for national parliaments to block EU laws, deregulation and an exclusion for Britain from the EU's "ever closer union".
Mr Cameron also wants to ensure full British access to the EU single market - preventing any protectionism by eurozone countries - acceleration of free trade deals and a halt to "vast migrations" from future EU accession states.
Although Mr Cameron's plans on migration could require treaty change - and are already being resisted by some central and eastern European countries - most of the demands could be met through political deals.
Mr Cameron has promised a referendum before the end of 2017 although ministers say a vote might be possible as early as the second half of 2016.
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