THE UK Government has submitted its full written case to the Supreme Court arguing that Holyrood cannot stage a second independence referendum under its existing powers.

The Advocate General for Scotland, Lord Stewart of Dirleton QC, has also asked the Court to consider whether it should refuse to look at the issue at all.

His submission addresses “preliminary issues as well as the substantive issue of [Holyrood’s legislative] competency.”

It follows the Scottish Government's top law officer, the Lord Advocate Dorothy Bain QC, asking the Court in June for a definitive ruling on whether Holyrood can hold Indyref2 without Westminster’s consent.

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.

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 if they wanted independence would not necessarily relate to the Union, as its direct legal effect would be “nil”, because it would not of itself change the law.

The SNP later 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 that Indyref2 per se would not have any effect on the Union, as it would be purely advisory, not self-executing.

The UK Government argues a referendum on independence would most certainly “relate to” the off-limits issue of the Union.

It is argues that, as a preliminary matter, the Court should refuse to hear the case, as it cannot reasonably pronounce on a draft Bill which could be amended.

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

The Scotland Office said the Advocate General has asked the Supreme Court for permission to publish his full written case as well as an earlier preliminary submission.

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 submitted our written case to the Supreme Court, in accordance with its timetable.

“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, who asked Ms Bain to refer the matter to the Court in the first place, has said that if the justices rule Holyrood cannot hold Indyref2 under its existing powers, 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.