When the Scottish National party published its roadmap for a second independence referendum on Sunday, it confirmed what many anticipate will be a central plank of its manifesto for May’s Holyrood elections. If the party wins a majority, as polling indicates it will, and if Westminster ignores that electoral mandate and continues to refuse the necessary transfer of powers, Holyrood will legislate to hold a referendum regardless. There are three possible routes to another referendum on Scottish independence. As happened with the 2014 vote, a process previously described as the “gold standard” by Nicola Sturgeon, the UK government could grant a section 30 order under the Scotland Act, 1998, granting Holyrood the competence to legislate for it. The act, which set up the Scottish parliament, states that Holyrood can’t normally legislate on reserved matters, such as the union. But UK prime ministers, most recently Boris Johnson, have repeatedly refused this request. Another, highly unlikely, route would be a Catalan-style wild-cat vote, which Sturgeon herself has said she could not sanction and would likely result in a boycott by unionist parties with significant damage to Scotland’s international credibility at a moment when it would also be seeking to rejoin the EU. Thirdly, as the SNP proposed at the weekend, it could introduce a referendum bill to the Scottish parliament without a section 30 order, prompting an almost inevitable legal challenge to settle whether holding a referendum on the union counts as a reserved matter and is thus beyond Holyrood’s competence, likely to end up in the supreme court. This is the “Plan B” pushed by independence activists increasingly frustrated at Sturgeon’s more cautious approach. One of the first tests in this scenario is for the member in charge of the bill, who has to state in terms that the bill is within competence on introduction, putting the lord advocate, Scotland’s most senior law officer, under significant pressure to intervene. The Scottish parliament’s newly elected presiding officer would have to confirm in writing whether a bill was legally competent. A government is not obliged to withdraw or change legislation that the presiding officer declares incompetent – as happened once before with the continuity bill aimed at upholding EU laws in Scotland after Brexit, which also went to the supreme court. But a ruling of that type would be likely to bolster a legal challenge. Once a bill is passed by Holyrood, the Scottish and UK governments’ law officers have 28 days to state whether they accept or challenge it. That may put the lord advocate under more pressure. If they challenge its legality, it automatically goes to the UK supreme court for a ruling – a process invoked with the continuity bill, which was ruled unlawful on technical grounds. The job of the supreme court, then, is to test the scope of the reservation in the Scotland Act, explains Kenneth Armstrong, professor of European law at the University of Cambridge. “Is the Scottish parliament competent to legislate for a referendum on independence even while the union remains reserved? The structure of devolution is such that everything is devolved unless it is reserved so it follows that the court ought to interpret reservations narrowly, ” he said. While Armstrong’s instinct is that the prospects of success for the Scottish government at the supreme court are low, he adds the caveat that the breadth of reservations in the Scotland Act allow greater freedom of interpretation. A case currently at the court of session tests this precise proposition. The crowd-funded action brought by independence activist Martin Keating is seeking a declaration that the Scottish parliament does have the power to hold a legal referendum on independence. Aileen McHarg, professor of public law and human rights at the University of Durham, has been following the case with some interest, but believes there is a strong chance that the court will simply accept the procedural arguments that the action is premature and not relevant. She notes that, from an SNP perspective, the timing of this case is unhelpful. “Assuming the SNP win a majority in May they will be in a much stronger position,” she said. “But a mandate alone, though politically important, isn’t legally relevant. If a court decides that the Scottish government does have power to hold a referendum it will obviously create significant political momentum. But they would still need the UK government consent ultimately to leave the union.” She makes the crucial distinction between a legal referendum and a legally binding one. “The 2014 referendum was advisory only, as was the Brexit vote, but it was legally valid because of the section 30 order.” McHarg also highlights the gamble the SNP would be taking by pursuing their case in court: if they lost, holding a referendum would then be “a non-starter”. “You can’t force local authorities to cooperate, there would be no proper control of spending or the campaign, and a potential boycott by voters.” Indeed, pursuit of a referendum through the courts marks a significant shift for Sturgeon herself. In January 2020 she warned that such a legal challenge “might actually set us back”, and she continues to insist on the need to have a process that is “legal and legitimate”. Here lies the risk in seeking legal answers to a political question, says Prof Michael Keating, director of the Centre on Constitutional Change. “If the supreme court says you can have an advisory referendum it’s still not legally binding. So in that sense it’s not a legal question but about how it can be resolved politically and whether Westminster thinks it can hold out.”