Cameron is due to announce plans to give Britons a straight choice on whether to stay in the EU in a referendum to be held between 2015 and 2018.
"All he's trying to do is to kick the can down the road and to try and get UKIP off his back," said UKIP leader Nigel Farage, whose party has climbed to third in some recent opinion polls, mainly at the expense of Cameron's Conservatives.
"That's really what today's speech is about," he told BBC Radio, agreeing his party were "the winners today".
Farage said most of Cameron's Conservative MPs as well as those from the other main opposition parties would support the prime minister's view that Britain should not exit the EU.
"I'm under no illusions. Winning this referendum, if and when it comes, is not going to be an easy thing. But I feel that UKIP's job starts today."
(Reporting by Michael Holden; editing by Guy Faulconbridge)