David Cameron has told his fellow European leaders they need to be "flexible and imaginative" as he demands reforms to the EU.
The PM was speaking alongside French President Francois Hollande during a tour of his European counterparts.
Mr Cameron wants to renegotiate the UK's relationship with Brussels ahead of an in/out referendum by 2017.
Foreign Secretary Philip Hammond said the PM would warn that the UK could exit the EU unless reforms were agreed.
During talks with Mr Hollande, Mr Cameron said "the status quo is not good enough".
"I believe there are changes we can make that will not just benefit Britain, but the rest of Europe too," he added.
The PM will hold talks with German Chancellor Angela Merkel and Polish Prime Minister Ewa Kopacz on Friday.
He has not set out in full detail the reforms he is pushing for, but they will include tougher rules to prevent migrants claiming benefits.
He also wants safeguards to protect the City of London in the event of closer eurozone integration and an exemption for Britain from the EU drive for "ever closer union".
Earlier, arriving in The Hague for talks with Dutch Prime Minister Mark Rutte, Mr Cameron said they would focus on growth, jobs and a "pro-enterprise agenda" as well as "my plans for European reform".
Speaking on BBC Radio 4's Today programme, Mr Hammond played down talk of the referendum being held before 2017, the date by which it has been promised.
He also said the UK government had received legal advice saying changes to EU treaties would be needed to secure Mr Cameron's reforms - something that has so far been resisted by other EU leaders.
The foreign secretary said: "We have a clear set of requirements. The prime minister is very clear in dealing with European Union counterparts - that if we are not able to deliver on those big areas of concern that the British people have we will not win the referendum.
"And we expect our European Union partners to engage with us in delivering a package that will enable the British people to decide that they think Britain's future is best delivered inside the European Union."
Asked if that meant the government would still recommend Britain stay in the EU even if it was not able to secure major reforms, Mr Hammond said: "If our partners do not agree with us - do not work with us to deliver that package - then we rule nothing out."
Mr Hammond said the talks process had just begun but the UK expected to secure a "substantive package of reforms" over the summer and into the winter months.
He said he wanted to negotiate reforms as quickly as possible but the timeline would depend on the "mechanism" set up for the negotiations.
He said "I don't think we've ruled out" having a referendum on a new deal next year but "what matters is getting it right rather than doing it quickly," adding that the government is "in the hands of our counterparts in the European Union".
French MP Christophe Premat, a member of President Hollande's Socialist Party, said the UK's referendum could not be used "as a threat".
He told BBC Radio 4's The World At One: "We respect that a referendum will be held in Great Britain, that's the destiny of the people in the UK. That's correct.
"But, at the same time, we can't use the referendum as a threat in order to have more space in the renegotiation. The method should be approached in another way."
The EU referendum bill, which has been published and was earlier presented to the Commons, contains details of the question and the commitment to staging the vote by the end of 2017.
Downing Street wants voters to be asked the question: "Should the United Kingdom remain a member of the European Union?"
The Electoral Commission suggested this form of words - which would make those campaigning to stay in the EU the Yes campaign - in 2013.
A spokesman said: "We will consider the contents of the Bill when it is published and will make our views known as it progresses through Parliament to ensure voters' interests are put first."
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:
Responding to the Queen's Speech, Labour's acting leader Harriet Harman said her party would back the referendum bill.
Outgoing Lib Dem leader Nick Clegg warned against complacency and called for Mr Cameron to lead the bid for Britain to stay in the EU with conviction.
UKIP leader Nigel Farage said the wording of the referendum question was "simple, straightforward" and "unambiguous".
He added: "However, that Cameron is opting to give the pro-EU side the positive 'Yes' suggests strongly that his negotiations are so much fudge.
"He has already decided which way he wants the answer to be given, without a single power repatriated."