The UK Telegraph, citing an unnamed source, says British PM Prime Minister David Cameron has privately conceded he may have to campaign for Britain to leave the European Union

He'll do so if he considers the result of his renegotiation with Brussels to be unsuccessful:

"He has said that if he is completely ignored, or if they give him nothing, he will campaign to leave" the Telegraph's source said.

"It's not something he wants to do, but in recent weeks he's made it clear it's no longer impossible."

Reuters have the report here, the Telegraph article is here too (from a few days ago)