David Cameron has suggested he expects all members of the government to back him over any EU deal.
Speaking at the G7 summit in Germany, he said he was confident he could secure sufficient reforms but indicated that ministers who did not support him would have to resign.
The prime minister also said he was taking a “very open-minded view” on the timing of the in/out referendum.
It comes after President Obama declared he wanted Britain to stay in the EU.
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.”
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.
At least 50 Tory MPs, including 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 on the public vote.
Steve Baker MP, who is co-chair of the group, said he believed some cabinet members would resign over issues surrounding EU membership.
“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,”
It comes after Barack Obama gave the strongest indication yet that Washington wants a Yes vote in the referendum on Britain’s membership of the EU.
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.”