- Video report by ITV News correspondent Carl Dinnen
Former Prime Minister David Cameron was intensely frustrated by Theresa May's fence-sitting in the run-up to the EU referendum, according to his former communications chief.
Sir Craig Oliver said Mrs May failed to support Mr Cameron on not one but 13 separate occasions, before she reluctantly coming "off the fence" for Remain - and only after a "visibly wound up" Cameron angrily spoke to her.
In his new book, Unleashing Demons: The Inside Story Of Brexit, which is being serialised on the Mail on Sunday, Sir Craig also describes Boris Johnson's "flip-flopping" on the weekend he came out in support of Brexit.
Sir Craig says that the day before announcing he would head up the Vote Leave campaign, the former London mayor warned Mr Cameron he would vote Out in a text, only to send a second suggesting he could change his mind.
While Mrs May's unwillingness may have antagonsied Mr Cameron, Sir Craig admitted that her approach has served her well.
"Amid the murder and betrayal of the campaign, one figure stayed very still at the centre of it all - Theresa May. Now she is the last one standing," he wrote.
He also described how Boris flip-flopped before the referendum.
"I ask DC what makes him so sure Boris is wobbling. He reads out some parts of the text including the phrase 'depression is setting in', followed by a clear sense that he's reconsidering. Neither of us is left in any doubt," he wrote.
"I am struck by two things: Boris is genuinely in turmoil, flip-flopping within a matter of hours; and his cavalier approach."
The day after, Mr Cameron received a text from Mr Johnson saying he would back leave - just minutes before publicly announcing his intentions.
Sir Craig wrote that Mr Cameron believed Mr Johnson was "really a 'confused Inner'".
A second book, All Out War, written by The Sunday Times political editor Tim Shipman and serialised in that paper, claims Mr Cameron branded Mrs May "lily-livered" after she ruined his plans for new immigration controls.
Mr Cameron wanted an "emergency brake" on migration as part of his renegotiation with the EU - to convince voters he could reduce immigration.
But according to the book he was blocked by Mrs May - who was not prepared to take on then-foreign secretary Philip Hammond, and German chancellor Angela Merkel.
It quotes one of Cameron's aides as saying: "Hammond spoke first and argued we just just couldn't do something that would receive an immediate raspberry in Europe.
"Theresa said very, very little, and simply said that we just couldn't go against Merkel."
A "visibly deflated" Mr Cameron was said to looked to an official and said: "I can't do it without their support. If it wasn't for my lily-livered cabinet colleagues...."
On Sunday allies of Theresa May denied she "let down" David Cameron over the EU referendum.
Asked on Sky News' Murnaghan programme if Mrs May had "let down" Mr Cameron, Conservative Party chairman Sir Patrick McLoughlin said: "I don't think that is true at all. Theresa May during the referendum campaign made her position very clear.
"This is a book that has been written after the event. You have got to have certain spicy things in a book to sell it. I don't blame Craig for doing that. At the time, Theresa was very much part of the Remain campaign."
However, former Northern Ireland secretary Theresa Villiers, who campaigned for Leave, said she believed Mrs May had been listening to both sides.
"There were times that I did wonder," she told BBC One's Sunday Politics programme.
"Her major speech of the referendum campaign expressed real concerns about the possibility of Turkey joining the EU. It also said that the sky is not going to fall in if we leave. I think she was genuinely listening to both sides."