Nick Clegg wants to "sign up to everything that comes out of Brussels without asking questions", David Cameron said in a heightening of hostilities ahead of May's European elections.
The prime minister accused his Liberal Democrat deputy of a slavish acceptance of European Union proposals as he declined to back his chances of winning a live debate with Nigel Farage.
Clegg is due to go head-to-head with the UK Independence leader in an hour-long radio session next week and another on television the week after.
Asked who he thought would win, Cameron said: "The only winner will be LBC radio, I'm sure."
Interviewed on the station, which will host the first debate, he went on: "The problem with this debate is that the right answer is neither take Farage's view and just leave Europe and be done with it or Nick's answer which is to sign up to everything that comes out of Brussels without asking questions.
"The right answer is to reform the European Union, get a better deal for Britain and then have a in/out referendum. There's only one prime minister that will give you that, and that's me."
A spokesperson for the Lib Dems told The Huffington Post UK that rather than having a clear position on EU, Cameron was "flirting" with eurosceptics for political reasons.
"The Conservatives are all over the place on Europe, with many of them flirting openly with exit. Nick Clegg is prepared to stand up for what he and the Liberal Democrats believe in - that we are better off IN," the spokesperson said.
"Only the Liberal Democrats will make the case for a stronger Britain in a modern, reformed EU, protecting millions of British jobs. We are standing up to the eurosceptics, not flirting with them."