THE warning is becoming louder. It was raised by the Leave team during Thursday's TV debate and, on the same day, by the Chancellor, George Osborne, and two former prime minsters, Sir John Major and Tony Blair. Brexit, they said, posed a serious threat to the Union.

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. But who is right? The question is as pressing for many Scots, on both sides of the independence debate, as the issue of the UK's EU membership.

Sir John qualified his remarks, saying a second independence referendum might not come straight away. The evidence suggests it wouldn't.

A poll this week asked how people would vote in 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 Nicola Sturgeon wants to see before she risks putting the questions to the people again.

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.

After fuelling speculation about the prospect of a post-Brexit indyref during the Holyrood election campaign, Nicola Sturgeon said in the ITV debate on Thursday she was "not here to speculate" about it.

Humza Yousaf, the transport minister, has also talked down the prospect in recent days.

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

In an earlier TV debate, staged by the BBC in Glasgow, Alex Salmond said he believed a second independence 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 foresees an independence campaign fought on the question of keeping Scotland in the EU.

Here, though, the crystal ball becomes very murky indeed.

Ms Sturgeon has said the "democratic outrage" of Scotland being forced out of the EU against its will would increase demands for independence.

A referendum in, say, 2018 would give the SNP time to work on its economic case and fight a campaign against a backdrop of EU withdrawal talks that - if you believe the Remain side - would not be going well for the UK. Neil Kinnock, in a speech to Labour supporters in Glasgow on Thursday, put it well. Arguing the UK would be in a weak negotiating position, he 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 Brexit talks might also damage the UK Government's ability to fight an effective No campaign.

But there are other factors at play.

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. And would a "hard" EU border have to be established between Scotland and England?

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

The SNP might prefer to wait and, as Dr Hughes suggested, seek to block Scotland's removal from the EU at Holyrood or in the courts, creating "a major political crisis".

It would depend on public opinion and, with so many factors at play, that is impossible to predict.

But Dr Hughes's conclusion is worth recalling in the light of this week's warnings: "Brexit may or not lead to a rapid second referendum but it would spark a deep political and constitutional crisis between England and Scotland."