THERE was, of course, always the suspicion. But now David Cameron indiscreetly – and given events of this week – perhaps stupidly has let the tiger out of the constitutional bag.

Namely, that he got no less than the head of state to help him out on the campaign trail during the 2014 independence referendum.

I have to admit that, when a BBC colleague ushered me aside and told me what our former Prime Minister had admitted to, my jaw dropped.

Of course, that our nonagenarian monarch is a Unionist came as no great surprise.

In his book and in the BBC documentary, Mr Cameron recalls how he was staying at Balmoral the weekend when – just days out from the independence ballot – a thundering newspaper poll dropped on the Royal doormat to herald how, from a regal perspective, those nasty separatist forces had pulled ahead.

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At the hillside barbecue on the Balmoral estate as the Duke of Edinburgh – never backward in coming forward – roasted grouse and sausages, the former PM recalled floundering as he tried to reassure his guests everything was going to be fine when he could barely reassure himself that it would be.

That opinion poll sent alarm bells ringing across Whitehall. Those with good memories will recall how panic quickly set in. Unionist leaders past and present swung into action. Cameron, Miliband and Clegg dropped everything and shot up to Scotland while Gordon Brown produced his famous Vow.

For her part, having received the Cameron message loud and clear, albeit possibly via two or three press secretaries and equerries, the Queen acted.

While attending a service at Crathie Kirk she slowly ambled over to some well-wishers and uttered her famous - or infamous – words, depending on your view, that she hoped Scots would think “very carefully” about the future when they voted in a few days’ time. The message was clear.

In the documentary, Mr Cameron sought to cover his back by stressing his plea of help to Her Majesty was discreet, indirect, and he was “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”.

It did. In his book, the former Conservative leader expressed his “delight” at the impact the Queen arching the Royal eyebrow had had.

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Noting how the media, understandably, had eagerly seized on the Royal intervention, Mr Cameron noted, with a deal of understatement: “It helped to put a slightly different perception on things.”

It also might be remembered just days after that fateful moment in September 2014, Mr Cameron was schmoozing with fellow Conservative Michael Bloomberg, the then Mayor of New York, when, still revelling in the afterglow of the No campaign’s victory, he let slip that after he had excitedly rang the Queen to tell her about the referendum result, she “purred down the line”.

The then PM apologised profusely for having been caught by media microphones confirming what we already knew: the Queen was pro-Union.

The timing of Mr Cameron’s indiscretion could not have been worse. Not only does it come as Scotland marks the fifth anniversary of the independence referendum but also as the UK Supreme Court engages in legal arguments about the constitution and reminding us how our current Prime Minister had dragged the Queen into a politically volatile row on Brexit.

But there is a difference.

When our constitutional monarch is asked by the prime minister of the day to prorogue Parliament, he or she can do little else but concede. Yet when the premier asks them to lend a hand in a political campaign about the future of the country, the sovereign is not bound by the constitution and can refuse. The fact the Queen did not will leave her open to all sorts of accusations from those less well-disposed than Mr Cameron to the Royal Family.

It came as no surprise Buckingham Palace did not want to go anywhere near this controversy but it did raise an eyebrow, Royal or otherwise, that the SNP, officially declined to comment.

But Alex Salmond, irrepressible as ever, had no qualms in letting rip.

He accused Mr Cameron of “begging” Her Majesty to intercede for the Unionist cause, an act he denounced as “totally improper,” but which showed just how desperate the then PM was in the final throes of the Scottish campaign.

The former FM noted how Scotland should always remember how Westminster did “not recognise any political rule-book,” although I do not recall the Yes campaign ever complaining the 2014 was rigged.

Mr Salmond signed off with a typical flourish, noting: “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.”

As Nicola Sturgeon returns from her Berlin trip to take First Minister’s Questions today, it will be interesting to see if the SNP leadership maintains its uncharacteristic silence on a raging constitutional row. Will a Royal eyebrow be raised? Just a quarter of an inch?