Was this the face Call-Me-Dave was pulling at the moment of the - alleged - incident? Photograph: Murdo MacLeod for the Guardian

Was this the face Call-Me-Dave was pulling at the moment of the – alleged – incident? Photograph: Murdo MacLeod for the Guardian

This is damning – by Gordon Brown’s former spin doctor, Damien McBride, who should know his subject very well.

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.

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.