A leading QC has argued the Scottish Parliament already has the power to legislate for a second referendum on independence without prior approval from the UK Government.
Aidan O'Neill, an expert in constitutional law, said the Scotland Act could allow for MSPs to legislate for an IndyRef2 - but said his conclusion would need to be tested in court.
The QC issued his opinion after being instructed by solicitors representing a pro-independence campaign group who are crowdfunding a potential legal challenge against the UK Government over its refusal to grant a Section 30 order.
The document, paid for by the Forward As One group after it raised £40,000 over the last month, said MPs or MSPs could seek a “declarator” from the Court of Session on the issue.
"I consider there to be good arguments to the effect that the Scottish Parliament have the power, under the provisions of the Scotland Act 1998 as they currently stand, to legislate for the holding of a referendum on Scottish independence," he said.
He added: "The matter has never been the subject of authoritative legal decision and is not, however, free from academic controversy."
Mr O'Neill did not issue an opinion on whether the UK Government was obliged to make a Section 30 order.
The First Minister has previously said she does not favour a consultative referendum and would want any future plebiscite to be arranged on the same terms as the 2014 vote.
Following publication of the legal advice, a spokesman for Ms Sturgeon said on Thursday: “First and foremost this is an issue of democracy and respecting democracy. Respecting people’s votes in elections.
“Ultimately this has to be settled by respecting democracy, respecting election results, and the position the UK Government are taking, we believe, is pretty unsustainable.
“The onus shouldn’t be on us to come up with a Plan B or a Plan C or a Plan Z, there is a perfectly well understood political and democratic precedent for how this is done, and that is the basis on which we published in December, following the precedent of 2014.”
In his opinion, published this week, Mr O'Neill said: "The devolved Scottish Parliament was undoubtedly established by the Union Parliament as a body to which decision-making powers had been delegated, that does not entail that the Scottish Parliament therefore owes its legitimacy to the Union Parliament - or indeed the UK Government.
"The Scottish Parliament was established as a democratically elected legislature with power to make general laws in Scotland. It is directly democratically accountable to the people of Scotland, over whom it exercises its legislative power."
He continued: "On specific issues of political or constitutional moment, however, it might be more appropriate for the legislature to make provision for direct consultation with, and voting by, the people on a specified single issue by way of a referendum. Regular elections and occasional referendums may be seen to be among the measures required to ensure continued democratic accountability in and legitimacy of a polity.
"Against that background, it would be surprising indeed if the Scottish Parliament were deprived of the power to hold referendums in Scotland. But the Scottish Parliament does indeed have power under the Scotland Act 1998, to hold referendums."
Academic opinion on the subject is divided.
Aileen McHarg, a Professor of Public Law at Strathclyde University told The Scotsman last year that the position on Holyrood staging its own vote is “genuinely unclear”.
“Quite a lot of people are saying it’s definitely unlawful," she said. "I think those people are overstating their case because there is a plausible argument to be made that it is lawful, but there is a plausible argument that it’s not.”
The Scotland Act which brought about the creation of the Scottish Parliament 20 years ago set out the areas which were reserved to Westminster. The ability to stage referendums is not listed among them. The debate hinges on the impact this has on the Union.
In a legal sense, both the Brexit vote and 2104 Scottish referendum were only “advisory”. So simply asking the question of Scots may not be seen as affecting the Union.
Can we distinguish it as something that would directly affect the Union?” added McHarg. “That’s the point on which there is no clarity and it can only really be resolved by the courts.”
The Edinburgh Agreement signed by Alex Salmond and David Cameron in 2014 was seen as the “gold standard” of staging referendums in the UK and Sturgeon has said she would want to secure this again.
Edinburgh University academic Dr Elisenda Casanas Adam told the paper in 2019: “The question of whether the Scottish Parliament would have competence to legislate for an independence referendum was left open. It was left unresolved because they reached an agreement.