NEXT week's EU referendum is not about Scottish independence, First Minister Nicola Sturgeon has been at pains to point out this week.

Yet the question of a second indyref has cast a long shadow over the Remain-versus-Leave campaign in Scotland.

Initially, when a Remain vote looked a safe bet, both Ms Sturgeon and her predecessor as SNP leader and first minister Alex Salmond, talked up the prospect of a re-run of the 2014 poll.

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Ever since the Nationalists' defeat, a scenario in which the UK opted to leave the EU, despite a majority of Scots wishing to stay, has been held up as a potential "trigger".

Only a few weeks ago, Ms Sturgeon said such circumstances would "almost certainly" lead to a second independence referendum. But in recent days, as it has become clear the vote is balanced on a knife edge, the First Minister has become more circumspect.

The "democratic outrage" that would inspire a surge in support for independence has become a decision "that many people in Scotland would think...was a bit unfair".

Humza Yousaf, the transport minister and leader of the SNP's own pro-EU campaign, also talk downed the chances of a second independence vote in an interview on BBC Radio 4's Today programme.

At the same time, however, other leading voices in the Remain campaign have begun to issue increasingly strongly-worded warnings that Brexit would pose a serious threat to the Union.

It was raised by the Leave team during STV's live debate a week ago, and, on the same day, by the Chancellor, George Osborne, and two former prime minsters, Sir John Major and Tony Blair.

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Speaking during a visit to Northern Ireland, Sir John put it most dramatically.

The "uncomfortable truth is that the unity of the United Kingdom itself is on the ballot paper", he insisted.

Leave campaigners immediately dismissed the warnings as "fantasy". With leading Nationalists now more cautious about the consequences of Brexit, they looked to have called the Remain camp's bluff.

But the picture - on both sides of the independence debate - is far less clear cut.

Sir John qualified his remarks, saying a second independence referendum might not come straight away.

The evidence suggests it wouldn't.

A poll by TNS last week asked how people would vote in a referendum called on the back of a decision to leave the EU opposed by a majority of Scots.

Discounting the don't knows, it found 44 per cent for Yes and 56 per cent for No, well short of the sustained 50 per cent to 60 per cent support for independence Ms Sturgeon wants to see before she risks putting the questions to the people again.

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There is also the question of the SNP's preparedness for another referendum campaign.

The party still needs to rebuild its economic case for independence and make key decisions about its currency policy.

But that is not to say a Brexit vote, against the wishes of most Scots, would not give a significant boost to the independence cause.

In the first live TV debate of campaign, staged by the BBC in Glasgow, Alex Salmond said he believed a second independence referendum would take place within two years if such a scenario materialised.

The former First Minister was referring to Article 50 of the Treaty on European Union, which says a state will formally leave the EU two years after giving notice to quit, unless negotiations are either concluded sooner or everyone agrees to extend the process.

Mr Salmond was looking ahead to a possible independence campaign fought on the question of keeping Scotland in the EU.

That scenario might hold some big advantages for the SNP.

A referendum in 2018 would be fought against a backdrop of EU withdrawal talks that - if you believe the Remain side - would not be going well for the UK.

Former Labour leader Neil Kinnock, in a speech to Labour supporters in Glasgow last week, put it well.

Arguing the UK would be in a weak position to negotiate new arrangements with the EU on trade and other issues, said: "If you are not on the strong side of the table, you are on the menu."

If he's right about the UK getting a raw deal, the prospect of remaining in the EU as an independent country might become more appealing to Scots.

Tough and exhausting Brexit talks might also damage the UK Government's ability to fight an effective No campaign.

The two year delay would also allow the SNP to address pressing questions about its economic case for independence.

But it is far from the clear the advantages for the SNP would outweigh the disadvantages of trying to make a Scotland in the EU independent of UK that was on its way out of the door.

Writing in The Herald earlier this year, Kirsty Hughes, of the Brussels-based Friends of Europe think tank, highlighted a number of issues that could make it harder for the SNP to make a winning case for independence.

By staying in the EU, Scotland might end up in a different trading regime from its main export partner. A "hard" EU border may also have to be established between Scotland and England.

Mr Yousaf appeared to accept such considerations when he said a post-Brexit referendum presented "additional challenges" and "makes the argument for independence very difficult".

If a week is a long time in politics, two years is an age, and it is impossible to predict how such factors might shift public opinion.

In the run-up to the Holyrood election, Ms Sturgeon said the polls would have to show a majority in favour of independence for a sustained period before she would consider calling a second independence referendum.

If Brexit failed to provide the surge the Nationalists require, the SNP could still seek to block withdrawal politically, at Holyrood, or legally in the courts.

Either move would create "a major political crisis" argued Dr Hughes.

She concluded: "Brexit may or not lead to a rapid second referendum but it would spark a deep political and constitutional crisis between England and Scotland."

There is another scenario which Sir John Major failed to mention but which Ms Sturgeon has: Scotland could keep England in the EU against its wishes.

The numbers are tight but, based on turn-out at the last general election, if 70 per cent of Scots vote Remain, something like 52 per cent of the rest of the UK could vote Leave and still be defeated.

The effect that might have on Scotland's constitutional debate can only be guessed at.