"If such negotiations lead to a conclusion the fruits will only be available to those who are members of the union," he told reporters in Berlin.
Even as he raised the specter of a referendum, Cameron reiterated his view that Britain should stay in the EU.
"I speak as British prime minister with a positive vision for the future of the European Union. A future in which Britain wants, and should want, to play a committed and active part," Cameron said. "There is no doubt that we are more powerful in Washington, in Beijing, in Delhi because we are a powerful player in the European Union."
In Washington, White House spokesman Jay Carney said the administration welcomes Cameron's call for Britain to remain an active force in the EU.
"We believe that the United Kingdom is stronger as a result of its European Union's membership and we believe the European Union is stronger as a result of having the United Kingdom in the EU," he said.
The timeline Cameron laid out mostly hinges on a Conservative victory in the next general election. But he said legislation will be drafted before 2015 so that if his party wins, it can be introduced and passed quickly to ensure a vote could be held "in the first half" of the next Parliament.
Cameron's proposals drew lukewarm support from the foreign minister of the Netherlands, where the prime minister was initially slated to give his speech last week before it was postponed due to the Algeria hostage crisis.
Frans Timmermans said in a statement that the Netherlands agrees with many of Cameron's criticisms of the EU.
"That's why we want to keep the British on board in the EU," he said. "Because you reform the EU from within, not by walking away."
Associated Press writers Elaine Ganley in Paris; Geir Moulson, David Rising and Frank Jordans in Berlin; Don Melvin in Brussels and Angela Charlton and David McHugh in Davos, Switzerland, contributed to this report.
Cassandra Vinograd can be reached at http://twitter.com/CassVinograd
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