The draft contained the referendum question, established the modalities for the poll, and even provided for the font of the print on the ballot paper. In a surprise announcement, the First Minister also noted that the Lord Advocate was simultaneously referring the legality of the measure to the Supreme Court.

This, Ms Sturgeon explained, was evidence of the determination of the Scottish government to comply with the rule of law.

“Respect for the rule of law means that a referendum must be lawful,” she proclaimed, “an unlawful referendum would not be deliverable. Even if it was, it would lack effect. The outcome would not be recognised by the international community.”

This point is clearly correct. Some other states would be reluctant to recognise Scotland’s independence unless it is the outcome of a consensual process.

At present, five EU members are obstructing Kosovo’s bid to join until Belgrade has accepted its secession. They would do the same if London is not fully supportive of Scotland’s statehood.

The SNP’s frustration is understandable. For over a year, Edinburgh’s attempt to schedule a referendum has been denied by the government in London. Boris Johnson flatly refused to issue a so-called Section 30 Order, allowing the Scottish Parliament to act in relation to a matter reserved for Westminster, and his successor would likely do the same.

The 2012 Edinburgh Agreement resulted in a Section 30 Order ‘allowing’ the 2014 referendum. This might suggest that a permission of this kind is required before Holyrood can legislate. But the document clarifies that its purpose was merely to “put beyond doubt” such authority. Hence, the question of whether the Scottish Parliament can adopt referendum legislation in the absence of agreement from Westminster was left unresolved.

Ms Sturgeon has emphasised the poll will be merely consultative. It is only meant to “ascertain the views of the Scottish people for or against independence”. Moreover, the referendum result is not “self-executing”.

Hence, it is argued, any outcome would not immediately affect the UK constitution or the future of the Union, items reserved for the Westminster Parliament.

The opposing view would assert that the very purpose of the referendum is to impel the UK’s constitutional organs to implement the result. This was suggested by the text of the Edinburgh Agreement, which said the referendum would be “a decisive expression of the views of the people in Scotland and a result that everyone will respect”. Presumably “everyone” would include the government and other constitutional organs, which would be expected to implement the result.

The First Minister has insisted the new referendum will be exactly like its incarnation of 2014. Indeed, if the result favours independence, the SNP will presumably very much insist on implementing the will of the people thus made manifest. This would then quite clearly affect the UK’s constitutional order.

If the Supreme Court accepts the reference, the outcome is therefore in some doubt.

A ‘no’ from the court would end the prospect of a referendum, but it would not end the matter. The Scottish government has asserted that a negative result would merely prove that the Union is not an equal partnership, aiming to disenfranchise the people of Scotland. Should this happen, Ms Sturgeon has announced that the SNP will run in the next general election on a single issue – independence.

This would turn the elections into a quasi-referendum. If successful, the UK government would be obliged to take account of the will of the Scottish people expressed in this way, it is argued.

This second, tough-guy tactic rather undermines the assurance given by the First Minister that the Scottish government will stay within the law. In fact, that commitment seems to hold good only if the law delivers the result the SNP is seeking. The argument seems to be one of “if we lose on the issue of legality, we still win based on the principle of democracy”.

Such a strategy would allow Number 10 to assume the position of defending the rule of law, rather than appearing to frustrate a legitimate expression of the wishes of the Scottish people. Moreover, the Scottish government would give up the very principle of legality which it has claimed is necessary to persuade other states to recognise Scottish independence and to grant EU membership.

In addition, by launching its preemptive reference to the Supreme Court, Ms Sturgeon risks reducing judgment on her campaign for independence to a technical debate about process and legislative competence. The real question should be whether the UK government would be entitled to withhold its consent for a referendum, if this is in fact required at all.

As the 2014 poll made clear, the UK constitutional order accepts that Scotland is a self-determination unit that can decide to leave. This is in contrast to the constitution of Spain, which positively ruled out Catalonia’s attempted secession. If the people of Scotland enjoy the right to determine their future through an act of will, then the UK government should not be able to deny the means of making that will manifest.

There is no international rule which holds that a referendum of this kind can only happen once in a generation. Instead, a fresh referendum after the passage of a decade or so seems reasonable. In addition, Brexit has fundamentally changed the situation.

The hope must be that the sides will still be able to come to an Edinburgh Agreement 2.0, perhaps after the expected change of guard at Number 10. In the meantime, those supporting the Union might finally focus on devising an attractive formula for the future of Scotland in the UK that might be presented as an alternative to independence.

Marc Weller is professor of international law and international constitutional Studies at the University of Cambridge. This article first appeared in The Scotsman.