David Cameron has said he expects all his ministers to back any deal he makes with the European Union or leave the government.
Speaking at the G7 summit in Germany, he said everyone in government had signed up to his plans as set out in the Conservative manifesto.
It comes after US President Barack Obama said he was “looking forward” to the UK staying in the EU.
And a group of 50 Tory MPs have formed a group pushing for radical reforms.
Mr Cameron’s plan is to renegotiate the UK’s relationship with the EU ahead of an in/out referendum by 2017.
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The prime minister said: “If you want to be part of the government you have to take the view that we are engaged in an exercise of renegotiation to have a referendum and that will lead to a successful outcome.
“Everyone in government has signed up to the programme set out in the Conservative manifesto.”
Mr Cameron added: “I am carrying out a renegotiation in the national interest to get a result that I believe will be in the national interest. I’m confident I can get that.”
He told reporters it was not a “on the one hand, on the other hand approach”.
“The government isn’t neutral in this. We have a clear view: renegotiate, get a deal that’s in Britain’s interest and then recommend Britain stays in it.”
The prime minister also said he was taking a “very open-minded view” on the timing of the vote, with some in the party keen for it to be held earlier.
‘Rather unwise’
BBC assistant political editor Norman Smith said the PM’s position was in “complete contrast” to the 1975 referendum on membership of the European Economic Community, when some cabinet ministers favoured withdrawal.
“This time, Mr Cameron is saying ‘whatever deal I strike, I expect my ministers to back me’,” he added.
Eurosceptic Conservatives have made it clear they are ready to campaign for an exit vote if the prime minister fails to come up with a package they believe delivers real change.
EU referendum in focus
David Cameron is starting renegotiation of the terms of Britain’s EU membership ahead of a referendum. Here is some further reading on what it all means:
Q&A: The UK’s planned EU referendum
The UK and the EU: Better off in or out?
What Britain wants from Europe
Timeline: EU referendum debate
Former Conservative minister David Davis said Mr Cameron’s stance was “rather unwise”.
He told BBC Radio 4’s Today Programme it was “pretty plain” that anyone wanting to vote to leave the EU would have to quit the government, adding that this was “something of a change from what was presumed right up to the election and, in my view, a rather unwise change”.
But communities and local government minister James Wharton, who as a backbencher piloted legislation for the EU referendum, said it was “reasonable to expect” the principle of collective ministerial responsibility to apply.
He said Mr Cameron had not ruled out recommending a No vote if his reform proposals are rejected.
Meanwhile, former cabinet ministers Owen Paterson and John Redwood have signed up to the newly-formed Conservatives for Britain group to keep the pressure on ahead of the public vote.
Steve Baker MP, who is co-chair of the group, said he believed some cabinet members would resign over the issue.
“If we don’t get a sovereign Parliament, I would be quite surprised if one or two don’t resign, but that really is a matter for them,” he told BBC Radio 5 live.
It comes after Mr Obama gave the strongest indication yet that Washington wants a Yes vote in the EU referendum.
During talks with the prime minister in Schloss Elmau, he said America was “looking forward” to the UK remaining part of the EU.
He added: “I would note that one of the great values of having the United Kingdom in the European Union is its leadership and strength on a whole host of global challenges, so we very much are looking forward to the United Kingdom staying part of the European Union because we think its influence is positive not just for Europe, but also for the world.”
On Twitter, UKIP leader Nigel Farage said: “We don’t need to take foreign policy advice from the American President. The last time we did that it was called the Iraq War.”
He added that Mr Cameron “clearly wants to keep Britain inside the EU under any circumstances”.
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