THE wait goes on for Nicola Sturgeon’s independence referendum bill. Yet, surely, it is coming. This time, even Nicola Sturgeon cannot get away with marching her troops back down the hill without even a Bill to show for all their troubles.

When it comes, it is inevitable that the Bill will pass. There will be a lot of noise, as there always is, but the Bill will sail through Holyrood. It will provide that there is to be a repeat referendum on independence. It will probably set the question (which, everyone anticipates, will simply copy and paste the question asked in 2014). And, even if it does not set the date, it may well set a period of time within which the referendum is to be held.

And what then? Inevitably the Bill will be challenged in court. UK ministers have the option of referring the Bill – before it can come into force – directly to the UK Supreme Court. I’m sure they will exercise this option but, even if they do not, a private party will challenge the lawfulness of the Bill in the Scottish courts. Such a case would start in the Court of Session and would no doubt end up in the Supreme Court. It would be surprising, however, if the UK’s law officers allowed that to happen.

My bet is that they will take the direct route, and that the Supreme Court will have to decide, for once and for all, whether Holyrood has the legislative power to enact a Bill that triggers an independence referendum.

Almost everyone familiar with the Scotland Act 1998 – which is the law that governs here – is of the view that the Supreme Court would rule that the Scottish Parliament has no such power and that the operative provisions of the Bill would be unlawful. In my view this verdict is more likely than not, but the matter is not as clear cut as some people would have us believe.

In order to be lawful, the Bill would have to clear two hurdles. And these, by the way, are the only hurdles that matter. Arcane arguments about the Declaration of Arbroath, the Treaty of Union, or the so-called sovereignty of the Scottish people would be thrown out by the courts. Such arguments are of interest to political historians, but they carry no legal weight. Supreme Court case law has made it plain that, when it comes to arguments about the legislative competence of the Scottish Parliament, the only tests that apply are those set out in the Scotland Act.

Before coming to the legal hurdles, one other myth needs to be dispelled. Whether or not the Scottish Government have a mandate to call a second independence referendum is, legally, entirely irrelevant. To the extent that governments have “mandates” at all in our constitutional arrangements, they have authority only to act within the law.

If the Scottish Government had an overwhelming majority in Holyrood and it introduced a Bill that was beyond the Parliament’s powers, its majority would count for nothing in law. Even democratically elected governments have to act within and subject to the limits of their legal powers.

So what are the two legal hurdles? The first is that Holyrood’s legislation may not relate to reserved matters. Holyrood has legislative competence over devolved matters only – if a matter is reserved to Westminster it continues to be for the United Kingdom to legislate on, and not for the Scottish Parliament. Whether a Bill passed by Holyrood relates to a reserved matter is to be determined by reference to that Bill’s “purpose”, having regard to its “effect in all the circumstances”. So says the Scotland Act.

Reserved matters are listed in the Scotland Act. Among them are “aspects of the constitution” including “the Union of the Kingdoms of Scotland and England”. Plainly, a Bill that sought to terminate the Union would be unlawful, as it would relate to a reserved matter. But Nicola Sturgeon’s Bill will be designed not to terminate the Union but to authorise a referendum on whether Scotland should be an independent country outside of the Union.

As such, it could be argued that its “purpose” is simply to ask the people of Scotland for their opinion; and it could be conceded that its “effect in all the circumstances” is actually very little. Referendum outcomes do not of themselves change the law. Brexit, for example, was not delivered by referendum. It was delivered by Acts of Parliament which took years of bitter negotiation and serial “meaningful votes” to enact.

Whether a Bill authorising an independence referendum impermissibly relates to reserved matters is a question on which no court of law has yet ruled. It is probable that the Supreme Court would rule that it did: probable, but not inevitable.

The second hurdle is that the Scotland Act provides that nothing Holyrood does may “affect the power of the Parliament of the United Kingdom to make laws for Scotland”. This is the provision on which the Supreme Court relied to strike down provisions of two Holyrood Bills passed in the last session.

In those cases the Court ruled that Westminster has “an unlimited power to make laws for Scotland”. This means that any Holyrood legislation seeking to place obstructions in the way of – or conditions on the way in which – UK law continues to have effect in Scotland is unlawful. Plainly this means that Holyrood could not legislate for independence. It is likely (but, again, not inevitable) that it also means Holyrood could not legislate for a referendum on independence.

These legal hurdles are formidable. But they are not insurmountable. Next week I’ll explain what the Scottish Government would need to do to overcome them, and what the implications of that would be for the likely effects and consequences of any repeat referendum on independence.

Adam Tomkins is the John Millar Professor of Public Law at the University of Glasgow School of Law. He was a Conservative MSP for the Glasgow region from 2016 to 2021.