David Cameron reveals he called European leaders and Barack Obama to say 'I'm sorry' on morning of 2016 referendum result

David Cameron has revealed how he called European leaders and former US president Barack Obama to apologise in the hours after the EU referendum.

The former prime minister said when the "enormity" of Britain leaving the EU dawned on him, he realised his failed effort to keep Britain in the bloc would "stay with me for the rest of my life".

In his memoir, set for publication on Thursday, he writes of feeling no choice but to resign following the 52-48 per cent defeat, and had to "muster what dignity I could" to hand in the keys to Downing Street.

He wrote that he was "sad to leave office but even more sad that Britain would be leaving the EU", when the "enormity of what happened" hit him on June 24, 2016.

"There were phone calls with the other first and deputy first ministers," he said. "I spoke to European leaders and to Obama. To each I said the same thing: 'I had a strategy to keep Britain in the EU. I executed the strategy. It didn't work. I'm sorry.'"

In extracts of his book being serialised in The Times, the Conservative also remembers calling his Leave rivals as Brexit became a reality, describing Michael Gove as "more shocked than anyone".

Another revelation in the book, titled For The Record, was that current PM Boris Johnson asked whether Mr Gove was "a bit cracked" after the Brexiteer betrayed him during the 2016 Tory leadership race.

Mr Cameron recalled the fraught battle to replace him in Number 10 in his memoir, casting back his thoughts to an "eerie" Westminster as Downing Street lay empty. "Power was fading like a dimming lightbulb," he wrote.

There were scenes of betrayal in Westminster as Michael Gove threw his weight behind Boris Johnson's campaign, only to dramatically withdraw his backing and declare he would stand himself - leading the now-PM to quit the contest.

The 2016 leadership battle was eventually won by Theresa May, and Mr Cameron revealed in the book how he secretly encouraged Gavin Williamson to help her campaign, according to The Times.

In extracts from the book published over the weekend, Mr Cameron took aim at Mr Gove, describing him as a "foam-flecked Faragist".

He said Mr Johnson wanted to become the "darling" of the Tory party and "didn't want to risk allowing someone else with a high profile - Michael Gove in particular - to win that crown".

In a stinging attack, Mr Cameron said the Prime Minister "didn't believe" in Brexit and only backed the Leave campaign to further his career.

He wrote: "The conclusion I am left with is that he risked an outcome he didn't believe in because it would help his political career."

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The former PM also struck a more raw tone in the book, recalling the death of his disabled first son Ivan, aged six, in 2009.

He praised the NHS for showing "​extraordinary compassion" in caring for his son, who was diagnosed with Ohtahara syndrome, and told there was no cure.

"A world in which things had always gone right for me suddenly gave me an immense shock and challenge," he said.

"Nothing, absolutely nothing, can prepare you for the reality of losing your darling boy in this way. It was as if the world stopped turning."

Additional reporting by PA