And he did not rule out a vote next year if talks went well but said what mattered was "getting it right".
Mr Cameron is flying to Holland and France on the first leg of a tour to build support for the changes he wants.
The prime minister has not set out in full detail the reforms he is pushing for, but they will include tougher rules to prevent migrants claiming benefits.
`Clear requirements`
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".
Mr Hammond 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 told BBC Radio 4`s Today programme: "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.
Referendum question
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".
First up in Mr Cameron`s European charm offensive are Dutch Prime Minister Mark Rutte and French President Francois Hollande, followed by Polish Prime Minister Ewa Kopacz and German Chancellor Angela Merkel.
The EU referendum bill, which is to be published on Thursday, 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 and vice versa - 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."
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