Andrew Parsons / PA Archive/Press Association Images

According to the book, one individual claims a pig was "resting on the lap of a Piers Gaveston society member while Cameron performed the act".

Ashcroft and Oakeshott say their source for the story is a current MP who was at university with Cameron. They say the politician has repeatedly made the accusation about the prime minister at a number of events in the last year.

The authors say this MP believes that photographic evidence of the incident exists and is held by a named individual. However, the person who is alleged to hold a photo of the supposed act did not respond to requests for comment from the authors.