Scotland’s lord advocate has confirmed she fears Nicola Sturgeon’s plans for a fresh independence referendum may be unlawful. Dorothy Bain QC has now released the letter she wrote to the UK supreme court last week seeking its ruling on whether Sturgeon has the legal powers to stage a referendum without the UK government’s authority. Bain told the court she is required to approve a statement by Scottish ministers when they table a new bill that it is legislatively competent under Holyrood’s devolved powers. “The lord advocate needs to have the necessary degree of confidence that a bill would be within devolved competence in order to ‘clear’ such a statement,” she told the supreme court. “In the present case, the lord advocate does not have the necessary degree of confidence.” Her admission confirms widely held suspicions at Holyrood that Bain has told Sturgeon that to stage an independence referendum without clearance from Westminster under a so-called section 30 order was likely to be unlawful. Boris Johnson has repeatedly said he would not grant that order. Sturgeon revealed last week she wants the powers to stage a referendum without it, despite significant legal opinion that only Westminster can approve legislation that affects the UK constitution. The lord advocate’s letter discloses the Scottish government believes a Holyrood-led referendum bill could be legal because it would only be advisory and has no legal force, and would pose a neutral question: “Should Scotland be an independent nation?” She told the court: “The Scottish government accepts that an act to dissolve the union is not within the legislative competence of the Scottish parliament. It does not necessarily follow, however, that the proposed bill, providing for an advisory referendum on independence, relates to reserved matters. “[Given] the advisory nature of the proposed referendum, it is questionable whether the proposed question (or the answering of it) purports to alter the fundamental principle of parliamentary sovereignty nor would it have this effect.” Prof Michael Keating, a constitutional expert, said Bain had clearly indicated she has doubts about the bill’s legality but was preserving her political neutrality by asking for the supreme court’s ruling. “It is unusual getting an advisory ruling from the supreme court, because that’s all this really is,” Keating said. “Normally you would wait until a bill has been passed before you refer it [to the court]. It seems to be very prudent.” Sturgeon said last week that if the court rules against the bill she would use the next general election as a “de facto referendum” where the Scottish National party would be empowered to open talks on independence if it won a majority of votes in Scotland. The Scottish Greens have since claimed votes for their party would count towards that total. Opposition parties were furious Bain’s letter had been published during Holyrood’s recess: they had asked her to appear before MSPs to answer questions on the supreme court application. Donald Cameron, the Scottish Conservative’s constitution spokesperson, said: “Yet again we can see exactly what the SNP are up to – playing political games by going to court in order to stir up grievance.”