Former PM David Cameron was prepared to stay on as Tory leader, but decided against it for fear of being led to the "slaughterhouse", a top aide has claimed.
Cameron's former director of communications Sir Craig Oliver also suggests that the current UK prime minister, Theresa May, was seen by some members of the Remain campaign as an "enemy agent".
In Oliver's book, Unleashing Demons: The Inside Story of Brexit, the former Tory aide says that May reluctantly "came off the fence" to support the Remain campaign during the EU referendum campaign, after Cameron became "visibly wound up" and rebuked her in tense phone conversations.
"Suddenly he picks up his mobile and calls May, asking her to make clear we have been victorious in our plan to crack down on 'swindlers and fiddlers' attempting to come into the UK," he wrote.
"When he hangs up he seems to think he's made an impact. Later the home secretary issues a statement saying she believes there's 'the basis for a deal here'."
Writing in the book which is serialised in the Mail On Sunday, Oliver describes the final days of Cameron's leadership: "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,"
Oliver also reveals that the current UK prime minister's nickname was 'Submarine May' as she often disappeared when Cameron needed her support, leaving him to "fight alone".
He stated that one of her excuses for not backing Cameron was taking a weekend holiday break with her husband.
While saying that Cameron felt "badly let down by May during the EU referendum, Oliver pays due to the current PM's strategy: "In terms of pure politics, she played it well, having her cake and eating it, but it doesn't seem fair on the PM who has treated her well."
Oliver, a 47-year-old former BBC news editor, worked with Cameron from 2011 until he resigned from office on 24 June.