AS Britain’s trick-or-treat Brexit nears, anxieties are growing on both sides of the constitutional argument.

If the UK crashes out of the EU without a deal at 11pm on October 31, no one knows quite what will happen but some Unionist politicians fear it could push Nicola Sturgeon to choose the “nuclear option” as a means of holding a second vote on Scotland’s future; collapsing her own government.

Frustrations among some Nationalists are running high, particularly in the wake of the birth of the Boris Johnson premiership.

The SNP’s MP Angus Brendan MacNeil and leading councillor Christopher McEleny have recently resurrected their bid to get their party’s autumn conference to back a route to UDI, a unilateral declaration of independence.

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The Western Isles MP admitted to being “utterly stumped” after the SNP conference committee rejected for debate at the party’s annual gathering in Aberdeen this October his and Mr McEleny’s plan to bypass what he regards as Whitehall’s intransigence against facilitating a second independence referendum.

Their proposal is simple: if the Nationalists won a majority of seats in a Westminster or Holyrood election, that should be enough to start negotiations over Scottish independence.

Of course, the idea of abandoning the notion that only another referendum could overturn the decision of a previous one is not new. Last October, the SNP’s Joanna Cherry mooted a “democratic event” would suffice ie if the SNP simply won the most Scottish seats at a general election, independence would follow.

Having had their bid rejected, Mr MacNeil and Mr McEleny are now trying a different route; tabling an amendment to an accepted conference motion on devolution.

It says a “pro-independence electoral victory shall be a mandate from the people of Scotland to commence independence negotiations with the UK Government,” if Westminster refused to grant the Scottish Government the power to hold a second referendum.

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Yet the First Minister wants any decision by Scotland to break free from the UK to be fair, legal and decisive, which was the basis of the 2014 vote as per the Edinburgh Agreement.

She insists on the back of the Holyrood vote in 2017, she has a mandate to hold a second referendum and has earmarked the latter part of next year to hold it.

But there is one big problem: London says no.

If the polls are right – two recent ones put the Tories 10 points clear of Labour – then should Boris Johnson make good his promise to lead Britain out of the EU in 74 days’ time, the chances of a snap general election are high. The Prime Minister will be banking on the backers of Nigel Farage’s Brexit Party swinging behind the Tories and giving him a working majority. The Tory manifesto would doubtless again include a line about refusing Ms Sturgeon’s desire to hold a second independence referendum.

Mr Johnson, just like Theresa May before him, will not agree to one because he does not believe the SNP Government has a mandate; it does not have a Holyrood majority and, until it does, all bets are off.

The Tory Government can do this because Westminster remains the constitutional authority.

The PM, given the turmoil of Brexit, needs a new battle on another front like a hole in the head, so will keep saying no. Indeed, if the Tories were to win a snap election in November, the next general election would not be due until late 2024.

So, is there anything Ms Sturgeon can do to get round this constitutional impasse? Possibly.

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Unionist MPs have spotted a potential albeit circuitous route, which is causing some of them a deal of concern.

To get to her desired destination, the FM needs that parliamentary majority in Edinburgh to knock aside Whitehall’s argument that the SNP administration does not have a mandate.

But the next Holyrood election is not until 2021. So, a second referendum next year is impossible, right? Wrong.

Section 3 of the 1998 Scotland Act has two ways to facilitate what is known as an “extraordinary general election” in Scotland. The first looks impossible; having a two-thirds majority of MSPs to call a new poll. This would mean getting most Labour MSPs to back the idea; which will not happen.

The alternative route, however, would involve Ms Sturgeon resigning her role.

There would be a month to find a new FM. But with the pro-independence Greens, the SNP has an overall majority at Holyrood and so could block anyone filling the role. An extraordinary general election would, therefore, follow. And, if Ms Sturgeon’s gamble paid off, the SNP would get its majority in the Scottish Parliament.

One could argue this is an extremely risky action because there are no guarantees the FM would secure a parliamentary majority.

But if Britain faces either a bad or a no-deal Brexit outcome with all the negative consequences people fear would follow, she might, in extremis, have to consider all possibilities, including the “nuclear option” of collapsing her own government.

If she got her new mandate, Ms Sturgeon could turn around to Mr Johnson and say: “Give me my referendum.”

Last week, even staunch Unionist David Mundell, the former Scottish Secretary, conceded that it would be "hard to push back" against a second poll if a Holyrood election was “fought explicitly on the issue of another referendum and then there is a majority of nationalist parties” for it.

The interesting reference was “nationalist parties”. At the last Holyrood election the SNP and Greens stood on different platforms but if they were to come together to put indyref2 front and centre – as they presumably would in an extraordinary general election to force another poll – then the chances of getting that nationalist majority would be so much higher.

In terms of time under this unorthodox route, if the FM resigned at the start of 2020, her desired second chance poll could indeed take place in the autumn of next year.

One Conservative MP noted that by collapsing her government, the SNP leader would “optimise her chances” of having a second poll. “If she gets a majority, I couldn’t see how the PM could refuse another independence referendum.”

A Liberal Democrat MP also raised concerns at the prospect, stressing: “This is why it’s so important to stop Brexit, so we don’t allow it to break up the UK.”

An SNP MP said the option of Ms Sturgeon resigning to force a snap Holyrood election was an “interesting route” to get to indyref2.

“She might do it,” he declared, noting: “The more unreasonable the UK Government is, the better. And they are being fantastically unreasonable at the moment. Something has to give.”

However, a senior Nationalist close to the SNP leader said while she was well aware of the Scotland Act option, it was “not in our thinking”. At the moment.

A UK Government source was dismissive. “If Nicola Sturgeon launched a cynical attempt to engineer another referendum, it would go down extremely badly with the majority of Scots, who do not want one in the foreseeable future.”

Of course, even if Ms Sturgeon were to win a majority in a 2020 extraordinary general election in Scotland, Mr Johnson could dig in his heels, raise his nose and simply say no.

And, indeed, if he were to win a snap UK general election on a manifesto of rejecting indyref2 until November 2024 at the earliest, he could simply respond to the SNP leader’s demand by saying: “Tough. My mandate’s bigger than yours.”