The relevant schedule of the Scotland Act states aspects of the constitution are reserved to Westminster such as the Crown, the parliament of the UK, and, importantly, "the Union of the Kingdoms of Scotland and England".

However, if the Scottish government takes the case for holding a referendum to court, it's likely to argue that the holding of an advisory referendum – just like the EU referendum of last year – has no direct legal effect on the union as it's not binding in law, and therefore not beyond Holyrood's power.

"The argument essentially is that the power to hold a referendum on independence is not included in the reservation of 'the union' to Westminster," said Aileen McHarg, a professor of public law at Strathclyde University.

"Holyrood can't legislate on anything that 'relates to' a reserved matter – so the question is, does a referendum relate to the union? Literally, yes, but statutes aren't always interpreted literally, and in determining issue, courts have to take account of the purpose and effect of a statute.

"Given any referendum would be advisory only, and, if there's another No vote, would have no effect on the union at all, the argument would be that it does not sufficiently relate to the union to be ultra vires [beyond Holyrood's power]."

Asked if such an argument could succeed, McHarg said: "It's arguable, and I'd say strengthened by the Supreme Court case [brought by Gina Miller], where the judges were all very clear that the effect of an advisory referendum was political not legal."

Professor Nick Barber, an expert in constitutional law from Oxford University, said: "The legal position is not clear, but I would take the view that the Scottish government could hold an advisory referendum on independence.

"Matters related to the Union are reserved, but whilst the Scottish parliament and government lack the power to make decisions within reserved areas it does not follow they lack the power to form opinions on these matters or to negotiate on these issues with Westminster.

"An advisory referendum on a reserved issue would be a way in which the Scottish institutions could demonstrate the strength of feeling in Scotland on the issue, and could play a part in that negotiation process."

However, Barber added of an advisory vote: "Westminster wouldn't be bound to accept the outcome – and the process might be weakened from the start by a declaration from Westminster that it would not recognise the outcome of the vote."