Nicola Sturgeon has been dealt a major blow in her plan to hold an independence referendum next year after her top law officer said she did not believe it was within Holyrood’s powers to call a unilateral vote.

A referral from Dorothy Bain, the Lord Advocate, was published on Tuesday, urging the Supreme Court to issue a ruling on whether holding a referendum without the permission of the UK Government would be within the Scottish Parliament’s legal competence.

However, in the document she conceded that she did “not have the necessary degree of confidence” to sign off on proposed legislation that would pave the way for Ms Sturgeon’s preferred option of a referendum on Oct 19 next year as constitutional.

The admission appeared to confirm speculation that a refusal by the Lord Advocate to approve the plan – required before Bills can be introduced at Holyrood – explains why the legislation had not yet formally appeared despite SNP claims that a vote would happen in less than 16 months.

Last week, Ms Sturgeon revealed that Ms Bain would ask the court to rule on the matter under an obscure provision of the Scotland Act that allows the Lord Advocate to refer “any devolution issue” to the Supreme Court.

Ms Bain said in her referral that she “needs to have the necessary degree of confidence that a Bill would be within devolved competence” in order to “clear” it for introduction at Holyrood.

In the case of Ms Sturgeon’s Referendum Bill, which would allow for a vote to be held with the same question as was asked in 2014, Ms Bain said: “In the present case, the Lord Advocate does not have the necessary degree of confidence.”

The First Minister has said that if her referendum plan is blocked by the Supreme Court she will attempt to transform the next general election into a “de facto referendum” on independence.

Opponents claimed the Lord Advocate’s comments added weight to claims that Ms Sturgeon’s plan was designed to stoke up grievance when her 2023 referendum is thwarted rather than a serious attempt at delivering a new vote.

Donald Cameron, the constitution spokesman for the Scottish Tories, said it was clear Ms Bain had been hidden from scrutiny because she was “not confident that the First Minister’s plan to hold a divisive and unwanted referendum has any legal basis”.

He 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.

“But strip away the legal wrangling and political posturing and this simply boils down to a SNP government focused on a self-serving, obsessive push for independence when they ought to be focused on the priorities of the Scottish people.”

In Ms Bain’s referral, she argues that it is important there is “clarity” on the question of whether Holyrood could hold a referendum without UK Government permission, saying it was a matter of “fundamental constitutional importance”.

She said the question had been “a source of debate” for 25 years, and that “opinion remains divided on the issue”.

However, since Ms Sturgeon set out her plans last week, experts have been near unanimous that Holyrood would not have the power to hold a meaningful vote without a Section 30 order, which was agreed by Westminster to allow the 2014 independence vote.

Some experts believe the court could even refuse to issue a ruling on the matter on the grounds that legislation has not yet been introduced.

Ms Bain acknowledged in her referral that the “Union of the Kingdoms of Scotland and England” and “the Parliament of the United Kingdom” were both matters reserved to Westminster.

The Scottish Government plans to argue that because a referendum would be “consultative” it could not in itself bring about independence and would therefore not be unconstitutional.

However, Ms Bain said in her submission to the Supreme Court that justices would consider “what the effect of the Bill would be in all the circumstances”.

The SNP has encouraged its supporters to believe there will be a referendum on Oct 19 next year. Michael Russell, the party’s president, recently launched a voter registration drive ahead of what he said would be a “historic democratic event”.

But Sarah Boyack, Scottish Labour’s constitution spokesman, said it was clear the Lord Advocate “does not have confidence that what the SNP is proposing is legal”.

She added: “With the country in the midst of a cost of living crisis, it is deeply disappointing to see Nicola Sturgeon return to the politics of the past.”

Neil Gray, an SNP minister, said: “There is a substantial majority in the Scottish Parliament in favour of an independence referendum and therefore a clear democratic mandate.

“However, as the First Minister set out last week, there remains debate over whether the Scottish Parliament has the powers to legislate to hold a referendum.

“A Supreme Court decision on the matter seeks to accelerate us to the point that we have legal clarity. We hope that it will be deemed to be within the legislative competence of the Scottish Parliament.”