David Cameron's communication boss, Sir Craig Oliver, publishes inside story of Brexit in new book

It was just after 7pm on June 23 when Sir Craig Oliver began to feel his stomach turn.

The communications director for former UK prime minister David Cameron was bruised and battered from a gruelling campaign when the sky turned black.

"I have felt confident of victory all day, but am beginning to feel uneasy," he writes in his new memoir.

Within hours, he walked out of Downing Street for one of the last times defeated.

"My mind feels calm ....my body seems to go into spasm. I suddenly retch harder than I have done in my life. Nothing comes up. I retch again - so hard, it feels as if I'll turn inside out.

"I lean forward on the rail and take a deep breath, my thoughts already turning to what went wrong and why?"

The visceral scene marks the opening chapter of his latest book, Unleashing Demons: The Inside Story of Brexit. Written in the three months since Cameron resigned following the Brexit vote that shocked the world, it's a compelling insider's account of how the vicious campaign played out behind closed doors.

It's packed with "raw history" from months leading up to the vote with behind the scenes details on key moments - from Boris Johnson's decision to back Leave to UKIP leader Nigel Farage's binned concession speech, Cameron's golf with Obama and Sir Craig's "God, I hate this referendum" text messages.

The former BBC editor-turned-spin-king said it's his chance to own up to mistakes made during the bitter campaign that many simply couldn't believe they had lost.

"We didn't think we were going to win by a massive amount but it was very hard to find anybody who thought we'd lost," he told news.com.au about the mood hours before the vote.

"The pollsters and the modellers and the hedge funds and the markets were all sort of assuming that Remain had won. The way I describe it is like feeling like you're walking on a path to safety and then you suddenly drop into quicksand and realise you're slowly sinking and that nothing and nobody is going to pull you out. It was shocking."

Sir Craig, who received a knighthood from Cameron in his last controversial honours list, said his team made two major mistakes; over reliance on the "iron law" of "it's the economy, stupid" and underestimating how motivated Leave voters were.

"We put all our chips on economic risk trumping all other arguments and actually what happened was that immigration became much stronger as an issue," he said.

"The second thing [was that] lots of pollsters and modellers said millions of people who hadn't voted in the 2015 election were very unlikely to vote this time .... They did. Nearly three million of them, by more than enough to ensure Leave's margin of victory. I don't think we focused enough on their concerns and irritations and we probably should have done."

With chapters like "She Could be PM in Six Months" about now Prime Minister Theresa May, "A Sticking Plaster on a Gaping Wound" and "This is Giving Me a Heart Attack", it's also a frank look at how the upper echelons of the UK political and media establishment operate that will prove shocking for some.

One chapter sees Sir Craig explain how he advised Cameron to reassure the public he would not resign following a Brexit vote, knowing full well his position could prove untenable. It's something he rationalises as the only option in an "impossible situation" that could have turned the entire campaign into a vote on Cameron's leadership.

"I appreciate that people don't see and get that nuance, but in politics you do. You have to get the nuance otherwise you're in trouble. When people thought about it, I don't think people were really that surprised he decided to go. You only knew the reality of the situation when you actually confronted it."

DIRTY 'DOGWHISTLE' POLITICS

Newly appointed Foreign Secretary Boris Johnson is described as a confused inner who dithered and voted Leave for political gain. Photo / AP

Sir Craig also takes aim at "ferocious" media coverage in the months leading up to the vote, with "deep frustration" at the BBC for airing Leave camp claims based on "dogwhistle" politics - such as Turkey being due to join the EU - that were made on air "without a proper slap down".

"[The BBC] get billions of pounds of public money in order to provide a service which actually dominates radio, television and online journalism in this country. For me, I don't think they spent enough attention thinking about the difference between balance and impartiality," he said, adding that the media exacerbated the binary debate.

"If you as a politician accept that you are uncertain of something or that your solution isn't perfect but it's probably the best thing in the circumstances, you will be eaten alive. You will be destroyed. You will be misrepresented and so what you do is you paint in primary colours or black and white."

'Submarine May' is how Oliver describes the now Prime Minister as she took a long time to decide on which way to lean. Photo / AP

Despite the tumultuous ride UK politics has taken since the vote, with a leadership change, the pound plummeting and warnings a "hard Brexit" could cost up to $107 billion a year in tax revenues, Sir Craig maintains it's a vote that had to happen.

"It split communities, it split families, it became a very difficult argument and I think it brought to the surface a lot of tension and poison that was already there," he said.

"I think what we probably thought was that it would drain a lot of the poison out and it would give people an opportunity to clear the air and get it over with. In fact it intensified things."