THE SNP’s new shadow Secretary of State for Scotland Mhairi Black has said “there could be mileage” in staging an unofficial independence referendum if Boris Johnson continues to rule out an official one.

She made the intervention in an interview as Nicola Sturgeon prepares to set out her government’s next steps in the independence campaign on Wednesday.

The First Minister has previously stressed it does not support an unofficial independence vote, which would be open to legal challenge and may not be recognised internationally.

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Earlier this month former Scottish Health Secretary Alex Neil said it was a route that should be considered if the Lord Advocate gave advice that such a vote would be legal. Black is the most senior figure to open the door to such a move.

She told the Sunday Times: “There could be mileage in a consultative referendum because the Scottish question is complicated.

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“The current United Kingdom Government position is weak and unsustainable, and if the last five years proves anything, it proves that what happens at Westminster is unpredictable.”

She added, however, that her preference would be to do the next referendum “by the book” and that a vote backed by law would ensure international recognition.

Asked whether she thought there would be a second referendum this year, she said: “I think so, although it might not suit the leadership to hear this, but I’m not fussed whether a referendum is this year or next year; we need to be able to take things as they come – as long as we can get one and get a majority for it.”

On shadowing and holding Scottish Secretary Alister Jack to account in the House of Commons, she rejected a suggestion from commentators as "a tussle between the proletarian and the patrician".

“I didn’t see it that way, I saw it more Jack against Black, two really different Scots with different backgrounds and different interests expressing themselves and their views. He strikes me as a decent chap,” she said.

Asked if she has had a chance to talk to him properly, she said: “I met him at Dover House last week and he was pretty straightforward.”

The National: Joanna Cherry

In a podcast for The Spectator, Joanna Cherry, the SNP’s Westminster spokeswoman on justice, suggested an independence referendum may be within the competence of the Scottish Parliament to hold an advisory referendum.

But she accepted such a vote “would then no doubt be challenged, its competence would be challenged in the UK Supreme Court and the Supreme Court would have to make a determination”.

As a republican, Black also set out her support for a referendum on whether Scotland should retain the monarch as head of state north of the border in the event of independence.

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“Personally, I’m not a monarchist, I’ve always been a republican and I would like to think there is a majority of Scots who take that view. Why is there a single family we all have to fork out for? Then there is the added baggage of all the privilege that surrounds it,” she said.

“It’s not my decision but in an independent Scotland I would support a referendum on the monarchy or some kind of vote.”

The First Minister will set out her “next steps” on Wednesday following Boris Johnson’s rejection of her request for powers over an independence referendum to be devolved permanently to Scotland.

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MSPs will also debate and vote on a Scottish Government motion calling for “Scotland’s right to choose” its constitutional future. With the SNP and the Greens holding a majority in Holyrood it is highly likely the motion will pass.

The debate will also put further strain on Labour which has come under pressure from some senior figures including MSPs Monica Lennon and Neil Findlay to back a new referendum.

Commenting on Black’s remarks, interim Scottish Tory leader Jackson Carlaw said: “There is a complete lack of discipline within the SNP but what is clear is that each and every one of them is intent on devising new and devious ways of keeping years of division alive around the independence question.”

Ahead of Sturgeon’s statement Scottish LibDem leader Willie Rennie will today make a case for reforming the UK. Rennie will say the UK establishment needs to understand the status quo is not sustainable and that voting reform, an elected second chamber, a written constitution and a federal UK is needed.