This story "hit the papers" shortly before October 2015 when the book Call Me Dave, a biography of the then-prime minister David Cameron was released. The book was written by Michael Ashcroft and Isabel Okeshott.
The book alleges that as a student at Oxford University, Cameron took part in some kind of initiation ritual which involved putting "a private part of his anatomy" into the mouth of a dead pig.
The story was told to them by an unnamed MP who claimed to have seen photographic evidence of the event.
Speaking on Channel 4 news (shortly after some excerpts of the book were published to the Daily Mail), Oakeshott said: "We couldn’t get to the bottom of that source’s allegations ... So we merely reported the account that the source gave us … We don’t say whether we believe it to be true". See this video.
In short, no supporting evidence has been produced. It is anecdotal to say, but it was likely a ploy to get publicity for the book.