As a spin doctor, there are two phone calls that make your heart sink: the one from a journalist relaying some excruciating allegation about your boss’s personal life or past history; and the second, the call you have to make to said boss in order to work out your response.

Working for Gordon Brown, a man of Victorian sensibilities and a volatile temper, the second call was invariably greeted with the single word “What?!” repeated with increasing volume and violence as I recounted the misdeed of which he had been accused. Even so, I’d conclude with the essential question that all spin doctors must ask in these situations: “What’s the truth?” Not, “What shall we say?” or, “How do you want me to handle it?” but instead the absolute insistence on knowing the full, unvarnished facts before deciding whether and how to spin them.

Sometimes, especially with Brown, that question provoked an angry barrage of abuse, as if just by asking it, I was implying the allegation might be true. That was good. That was what I wanted to hear. With other politicians, celebrities and friends I’ve advised over the years, you’d instead hear a dread pause, then a hesitant, “Well…”. That’s when you know you’re screwed.

So let us pause for a moment to pity the poor soul who had to call up David Cameron yesterday and tell him that – inter alia – the new biography of him by Lord Ashcroft and Isabel Oakeshott would allege that he’d once engaged in a bizarre ritual with a dead pig.

So what’s the truth? Only the prime minister and the porker know for sure, and neither have commented to date. But, in their place, we have a host of unofficial denials from “party sources” or “friends of Cameron”, and some well-informed scepticism from his Oxford contemporary Toby Young.

However, I’m confused by the lack of an official denial. Not from the No 10 civil service press office – that’s not their job – but from the many political advisers who work on Cameron’s media team. I regularly issued statements from No 10 as a “spokesman for Gordon Brown” on matters personal, and did so precisely when I needed a denial or clarification to carry extra weight, especially for the benefit of other media outlets considering whether to follow up the story.

Perhaps the spin doctors' “what’s the truth?” conversation with the PM wasn’t as clear cut as they might’ve liked

If, on Sunday night – when details first emerged from the Daily Mail serialisation – a spokesman for Cameron had issued an official statement saying: “This disgusting story is a complete and utter fabrication, and casts huge doubt on the credibility of all the other allegations in this book”, it would have been pretty devastating for the Mail and for Ashcroft, and would have deterred other newspapers from repeating either the pig story or the book’s many other revelations.

Indeed, the first thing you do as a spin doctor when a book is serialised about your boss or your party is to look for one howling error that you can highlight to discredit all the other accurate things the author has written, and suggest they’ve been relying on sources inclined to make things up.

So why no official denial in this case? Perhaps the PM’s spin team decided that responding officially to the pig story would oblige them to respond to potentially more damaging allegations regarding Ashcroft’s non-dom status. Better, instead, to say they were not commenting on any of the book’s contents.

If so, it was a major miscalculation, and one I’d suggest they are too experienced to have made. In the absence of an instant bucket of water, the story has caught fire over the past two days. Not only that, it’s allowed other newspapers to declare open season on Cameron’s private life, as we see from today’s “coke parties” splash in the Sun.

My successors in No 10 must have seen that coming, so their failure to head it off must be ascribed to something else.

Perhaps the “what’s the truth?” conversation with the prime minister wasn’t as clear cut as they might have liked, and they had no option but to let the story run, let everyone have their fun, and get on with the business of governing. Better that than officially denying the story and creating a much bigger problem for themselves if the alleged photographic evidence of the pig incident did emerge.

Hence the official line that the prime minister’s spokeswoman issued yesterday, saying they would not dignify the book’s claims with a response, almost word-for-word the quote I used routinely in my old job when responding to an allegation in a book that I knew to be true, for example Robert Peston’s claim that Brown had told Tony Blair that he would never believe another word he said. I learned back then what Cameron’s media advisers appear to have learned this week: sometimes a story is impossible to spin.