David Cameron became intensely frustrated at Theresa May's unwillingness to declare her intentions in the run-up to the EU referendum campaign, according to his former communications chief.

Sir Craig Oliver said Mrs May failed to support the former Witney MP on 13 separate occasions before she did reluctantly "come off the fence" for Remain - and then only after a "visibly wound up" Prime Minster gave her a dressing down over the telephone.

Sir Craig said Mrs May's unwillingness to declare her hand in the run-up to the campaign had caused frustration in No 10, but he admitted that her low-key approach served her well in the aftermath of the vote for Brexit.

In his book, he wrote: "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 describes one conversation with Mr Cameron in January - six months before the referendum - after the then Prime Minister had sought to sound out Mrs May - who was then home secretary - about her views on the EU.

Sir Craig wrote: "It sounds like she refused to come off the fence.

“From her point of view it's a smart strategy, trying to demonstrate she is her own person, allowing her to have her cake and eat it, but it doesn't seem fair on DC, who has treated her well.”

There was further consternation within No 10 when during a Cabinet discussion on the issue, Mrs May did not join in, "playing her cards close to her chest".

He wrote: "Her sphinx-like approach is becoming difficult, with the press questioning which way she will jump. The conversation turns around this being the biggest thing the PM has faced and him not even knowing if the home secretary is backing him.”

Matters finally came to her after a report in The Times warning that Mr Cameron faces "last minute opposition" from Mrs May to his deal on EU reform.

Sir Craig also describes how the day before he came out for Brexit, Boris Johnson, the former Henley MP, sent a text to Mr Cameron warning him that he would be campaigning for Leave, only to send a second message suggesting he could back Remain.

The book reads: "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.

"I am struck by two things: Boris is genuinely in turmoil, flip-flopping within a matter of hours; and his cavalier approach."