HOLYROOD “plainly” does not have the power to hold a second independence referendum without Westminster’s consent, the UK Government’s top law officer on Scotland has said.

The Advocate General, Lord Stewart of Dirleton QC, said that was true whether or not any referendum was advisory or directly changed the law.

He also said the “unavoidable” reason the Scottish Government wanted to ask voters about independence would be “termination of the Union and the secession of Scotland”.

Indyref2 would not be, and would not be designed to be “an exercise in mere abstract opinion polling at considerable public expense”.

He said: “Were the outcome to favour independence, it would be used (and no doubt used by the SNP as the central plank) to seek to build momentum towards achieving that end.

“It is in precisely that hope that the Draft [Referendum] Bill is being proposed,” he said, adding there was “an air of unrealistic casuistry” about the Scottish Government’s position.

As such, the referendum would improperly “relate to” the reserved issue of the Union and so be outwith the legislative competence of the Scottish Parliament.

Lord Stewart made the points in the UK Government’s written case to the Supreme Court on the matter, which was published this morning.

The Scottish Government's top law officer, the Lord Advocate Dororthy Bain QC, asked the court in June for a definitive ruling on whether Holyrood could hold Indyref2 without the loan of extra powers from Westminster, as happened in 2014.

The Scotland Act 1998 which underpins devolution says the Union is reserved to Westminster, and that any Holyrood legislation that “relates to” it is therefore invalid.

READ MORE: UK Government submits case against unilateral Indyref2 to Supreme Court

After refusing to sign off a Scottish Government draft Indyref2 Bill because she was not confident it was within Holyrood’s powers, Ms Bain asked the court to settle the matter.

Her written case was carefully balanced, but suggested that Holyrood legislation simply asking people their view on whether they wanted independence would not necessarily relate to the Union, as its direct legal effect would be “nil”, as it would not of itself change the law.

The SNP has asked to intervene in the case to make a more forceful argument that Holyrood already has the power to stage Indyref2, although it too argued a referendum would not have any effect on the Union, as it would be purely advisory, not self-executing.

In his submission, Lord Stewart said the Scotland Act already provided a mechanism for the Court to assess the legislative competence of Bills passed by Holyrood before they received Royal Assent and became law.

He said the Scotland Act’s framers did not intend to create two routes to the court, with the Lord Advocate asking justices to consider Indyref2 as a “devolution issue” a misstep.

He said the Court should dismiss the Lord Advocate’s reference as premature and misguided, noting the draft referendum bill could be amended at Holyrood.

“In the context of the present Reference, if the Court has jurisdiction it should nonetheless refuse the Reference in its inherent discretion to decline to determine abstract and premature issues in connection with a draft of a Bill which has yet to be introduced into and yet to be passed by the Scottish Parliament,” he said.

On the substantive issue of whether Holyrood had the competence to hold a referendum on independence, Lord Stewart argued it categorically did not under its existing powers.

This was because the issue “relates to” the off-limits matter of the Union, regardless of whether the referendum was purely advisory or not.

He said: “It is submitted that the Scottish Parliament plainly does not have the competence to legislate for an advisory referendum on the independence of Scotland from the United Kingdom, including in the form adopted in the Draft Bill. To do so impermissibly relates to both §1(b) and §1(c) of Schedule 5 to the 1998 Act “The effect of the Draft Bill is significant. The legal effects of it are not ‘relevantly, nil’, as the Lord Advocate’s Case suggests.

“On the contrary, legislation for a referendum on independence is required precisely because statutory authority is necessary to authorise the expenditure of significant financial resources on the exercise, and to direct the performance of the functions of counting officers and registration officers to conduct the referendum.

“It is, of course, right that the outcome of the referendum provided for by the Draft Bill has no legal effect: it is not ‘self-executing’.

“But nor can it credibly be suggested that the outcome of the referendum will be ‘advisory’ in the sense of being treated as a matter of academic interest only: a referendum is not, and is not designed to be, an exercise in mere abstract opinion polling at considerable public expense.

“Were the outcome to favour independence, it would be used (and no doubt used by the SNP as the central plank) to seek to build momentum towards achieving that end: the termination of the Union and the secession of Scotland. It is in precisely that hope that the Draft Bill is being proposed.

“It is not inappropriately speculative to have regard to that as an aspect of the effect of the Draft Bill: it is unavoidable.”

He went on: “It is difficult rationally to suggest that a Draft Bill which makes provision for a referendum on independence – on the ending of the Union – has only a loose or consequential degree of connection with the reserved matter of that Union.”

If the Court agrees to hear the Lord Advocate’s case, it will do so on October 11 and 12.

A UK Government spokesperson said: “People across Scotland want both their governments to be working together on the issues that matter to them and their families, not talking about another independence referendum.

“We have today [10 August] published the papers we have submitted to the Supreme Court, and will set out our case at the hearing in October.

“On the question of legislative competence, the UK Government’s clear view remains that a Bill legislating for a referendum on independence would be outside the legislative competence of the Scottish Parliament.”

Nicola Sturgeon has said that if the Court rules against her she will fight the next general election as a "de factor referendum" on the single question of independence. The Scottish Greens have said they would do likewise.

Responding for the SNP, MP Joanna Cherry QC said: "The British Government wants the Supreme Court to throw the case out without even hearing the substantive arguments.

"Their written argument exposes the fact they no longer believe the UK is a voluntary union of nations, instead, they want to impose Westminster control by denying democracy and dismissing the democratic right of the people of Scotland to determine our own future.

“Scotland voted to hold an independence referendum, and the Scottish Government has a mandate to deliver that manifesto commitment - with the backing of the Scottish Parliament.

"A clear precedent was set by the negotiations leading to the 2014 referendum, when there is a majority in the Scottish parliament to hold an independence referendum the two governments should come together and negotiate the details.

"The Tory government is desperate to prevent a referendum because it fears the result and has run out of any positive arguments for Westminster control.

"By arrogantly dismissing Scotland's democratic decisions, Westminster is making the case for independence stronger."

Scottish Liberal Democrat leader Alex Cole-Hamilton said: "The fact that a host of civil servants, the attention of ministers and some of the country's smartest legal minds are all currently focused on this court case rather than on the unfolding cost-of-living crisis is simply mind-boggling.

"I hate to think what the final bill will be.

"Scotland deserves better than two governments who would rather get at each other's throats over the constitution, than take action to relieve hard-pressed households.

"We need urgent action to cut energy bills and insulate homes but instead Nicola Sturgeon and Boris Johnson are asleep at the wheel."

Pamela Nash, chief executive of Scotland in Union, said: “In the week that the SNP’s shameful record on education has been exposed, it’s clear once again the nationalists’ attention lies elsewhere.

“Instead of pursuing her selfish obsession through the courts, Nicola Sturgeon should be concerning herself with matters closer to home.

“The appalling attainment gap is widening on her watch. It exposes her pledge that education is her number one priority as a complete nonsense.

“If she spent half as much time on the standards of education as she does on trying to break up the UK, pupils and teachers would be far better off.

“Scotland’s positive future is with the rest of the United Kingdom working together on the issues that really matter.”