David Cameron secretly asked the Queen to back the Union in the 2014 Scottish independence campaign

In a BBC documentary about his time in office, Mr Cameron admitted the snapshot in a Sunday newspaper just 11 days before the historic vote was “like a blow to the solar plexus” and led to a “mounting sense of panic that this could go the wrong way.”

He tells the programme: “I remember conversations I had with my Private Secretary and he had with the Queen’s Private Secretary and I had with the Queen’s Private Secretary, not asking for anything that would be in any way improper or unconstitutional but just a raising of the eyebrow even you know, a quarter of an inch, we thought would make a difference.”

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A few days later, the Queen famously told a well-wisher outside nearby Craithie Kirk that she hoped “people would think very carefully about the future”; regarded at the time and since as Her Majesty urging Scots to stick with the Union.

In an interview at the weekend about his autobiography, For The Record, Mr Cameron said how he had been “delighted” by the Queen’s intervention.

He tells the BBC documentary: “It was certainly well covered [by the media], although the words were very limited. It helped to put a slightly different perception on things.”

Shortly after the referendum result on September 18 2014, the ex-PM was caught on a TV camera during a visit to New York, telling Michael Bloomberg, the then city’s mayor, how Her Majesty had “purred down the line” when he informed of the referendum result.

Mr Cameron later apologised to the Queen, admitting he had felt “extremely sorry and very embarrassed” for confirming she had wanted Scotland to remain in the UK.

Today’s bombshell revelation comes in the week that marks the fifth anniversary of the referendum result - when Scots backed maintaining the 300-year-old Union by 55 per cent to 45 per cent - and when the UK Supreme Court deliberates on whether Prime Minister Boris Johnson’s decision to suspend Parliament for five weeks was lawful and which many politicians protested had dragged the Queen into politics.

Last night, Alex Salmond, the former First Minister, said: “Begging a constitutional monarch to make a political intervention is not only totally improper but an indication of how desperate Prime Minister Cameron was in the final stages of the Scottish referendum campaign.

“Scotland should always remember that Westminster does not recognise any political rule book,” declared the ex-party leader.

He added: “Cameron started the campaign uber-confident and ended up in a blue funk. I doubt If Scotland will let the Establishment off the hook next time around.”

The SNP declined to comment but a senior source said: “We have tried not to drag the Queen into political things; others may or may not do.”

A spokeswoman for the Queen told The Herald: “It’s not something we are commenting on at all.”

Mr Cameron’s highly controversial admission does not appear in tonight’s first instalment of The Cameron Years on BBC 1 at 9pm, "A Huge Fight on My Hands," but in the second part of the programme, “The Best is Yet to Come,” broadcast next week.

In his memoirs, For The Record, Mr Cameron recalled the tense run-in to the independence vote and his stay at Balmoral when at a hillside barbecue members of the Royal Family “gingerly asked questions but knew they shouldn’t express too strong an opinion”.

The next morning, conversation turned to the opinion poll. “I tried to reassure them about ‘rogue polls’ but I was struggling to convince myself, let alone them,” wrote the former PM.

Mr Cameron, whose late father Ian was from Aberdeenshire, made clear that he would not have resigned if he had been the prime minister to lose the 300-year-old Union but noted how he felt “if I were the one who precipitated the end of our island story, it would hurt beyond belief”.

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In tonight’s programme, George Osborne, the former Chancellor, reveals the depth of his opposition to Mr Cameron calling another referendum on Britain’s membership of the EU.

He says he is “very sorry for what happened” and admits he feels “responsible” because of his role at the Treasury, adding that “we held a referendum we should never have held” and the “consequences for the country are grave”.

He openly criticises the former PM’s approach saying: “David Cameron was just one of a number of British Prime Ministers who had fed this idea that we were different than Europe, that Brussels was to blame and that the public ultimately had to have a say, and we’ve all paid a price for it.”

Mr Osborne, now the editor of the London Evening Standard, points out the “only thing I can plea in my mitigation is…I made the case against it but it wasn’t heard”.

Mr Cameron and his then Chancellor also speak openly about the fraught conversations they had in the Downing St flat, trying to convince their then friend and colleague Michael Gove to come down on their side for Remain.

The Cabinet Office Minister, who became a leading figure in the Leave campaign, explains how he felt “some of the conversations we had were attempts on his part[Mr Cameron’s] to reassure himself that our friendship would mean that I wouldn’t stray from the fold”.

The Scot goes on: “David understandably felt that since I’d been prepared to knuckle under on a number of occasions beforehand and put my own feelings to one side in order to serve the team, that on this occasion, that I would do the same.”

Mr Cameron, who has branded Mr Gove a disloyal liar, describes as a “bombshell” the moment when he confirmed that he would support the Leave campaign.

Mr Osborne recalls the moment he warned his former Cabinet colleague, telling him: “If you go to the Leave campaign, you will destroy this Government; if we lose this referendum, David Cameron will have to go, there’s no way he’ll be able to survive, and everything we’ve worked on will fall apart.”