Cameron has pledged to hold a referendum by the end of 2017 at the latest on whether Britain should remain in the European Union, but said it could be held sooner than that.
If he remains prime minister after May, the Conservative leader intends to attempt a renegotiation of the terms of Britain's membership then seek consent for that settlement through a referendum.
"The referendum must take place before the end of 2017," Cameron told BBC television.
German Chancellor Angela Merkel reportedly told Cameron in October that he was approaching a "point of no return" with the EU over a proposal to limit the migrant intake from other member states.
In a speech in late November, Cameron promised tough curbs on welfare for EU migrants but stopped short of calling for a cap on new arrivals.
Cameron is due to hold talks with Merkel in London on Wednesday, when they will discuss European issues.
"The most important thing of all is being able to make changes to the welfare system," Cameron said in an interview with The Mail on Sunday newspaper.
"The key areas are safeguarding the single market, getting out of ever closer union, being able to veto regulation and a package of measures on welfare.
"If you look at the reaction to my welfare speech in Germany and one or two European capitals, you will see they gave it a broad welcome. Germany wants Britain to stay in Europe," he said.
With five months to go, the opposition Labour Party is ahead in the opinion polls.
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