On The Sunday Politics Scotland today, the First Minister of Scotland, Nicola Sturgeon, raised the prospect of Scotland placing an obstacle in the path of Brexit, saying: “If the Scottish parliament is judging this on the basis of what’s right for Scotland, then the option of saying we’re not going to vote for something that’s against Scotland’s interests, that’s got to be on the table. You’re not going to vote for something that is not in Scotland’s interests.” So, can Scotland legally block Brexit? The answer is “no”; but before the reasons are explained, some background is necessary.

Devolution notwithstanding, the UK Parliament — i.e. the Westminister Parliament — is sovereign. Among other things, that means that Westminster has not given away any of its powers to devolved legislatures like the Scottish Parliament. As a matter of strict law, the UK Parliament has merely authorised the devolved legislatures to make laws on certain matters, without relinquishing its own authority to make law on any matter it chooses — including devolved matters.

Of course, if the UK Parliament, having established devolution, regularly interfered in devolved matters, that would undermine the whole system. But in fact that does not happen, because the legal power of the Westminster Parliament to interfere is usually exercised by reference to a “constitutional convention” — an established and accepted political practice concerning how Westminster will and will not use its power — known as the Sewel Convention. The Sewel Convention says that the UK Parliament will not normally legislate on devolved matters, or on matters affecting the extent of devolved legislatures’ legal powers, without the consent of the devolved legislature. In other words, Westminster will not normally interfere in matters that affect or are within the Scottish Parliament’s authority unless the Scottish Parliament agrees. Importantly, for present purposes, the Scottish Parliament’s powers are limited by EU law. And the argument is that if Brexit legislation enacted by the UK removes those limits — freeing the Scottish Parliament to make Scottish laws that breach EU law — then that alters the Scottish Parliament’s powers, so triggering the requirement to get its consent under the Sewel Convention.

It might seem, therefore, that Scotland can block Brexit by withholding its consent under the Convention. However, for three reasons, the true position is that Scotland cannot legally block Brexit.

First, it is logically impossible for there to be any requirement for the Scottish Parliament’s consent Brexit legislation enacted by Westminster unless, in the first place, the Westminster Parliament is in the process of enacting such legislation. The general view, though, is that the Article 50 process — whereby the UK’s departure from the UK would be negotiated — falls to be triggered by the Government exercising its so-called prerogative powers to conduct foreign policy, rather than by Parliament enacting legislation. This means that when (or if) Article 50 is invoked, that could happen without any legislation being enacted by the UK Parliament. What this boils down to is that Brexit could become irrevocable (unless the EU agrees otherwise, it follows automatically two years after Article 50 is triggered) without the UK Parliament ever enacting any legislation to which the Scottish Parliament could object. Of course, at some point, the UK Parlaiment will have to enact legislation relating to Brexit — but if, by then, the Article 50 clock has begun to run, no amount of withholding of consent by the Scottish Parliament will make the UK’s departure from the EU any less inevitable as a matter of international law. That ship will have sailed.

Second, the Scottish Parliament cannot anyway “block” UK legislation on Brexit or on anything else. Certainly, it can withhold consent. But because, as explained above, the UK Parliament is sovereign and can do as it wishes, the absence of consent from the Scottish Parliament would not legally disable Westminster from enacting Brexit legislation. This is so because the “requirement” for consent is not a legal requirement at all: it is, ultimately, no more than a political expectation that the UK Parliament will respect the constitutional position of the Scottish Parliament by not riding roughshod over it in certain circumstances. It might be politically difficult — and highly inexpedient — for the UK Parliament to treat the Scottish Parliament in that way. It might even be thought to be “unconstitutional”, in the sense of disregarding the constitutional principle — namely, respect for devolved autonomy — that underpins the Sewel Convention. But because the requirement for Scottish consent is, at the end of the day, no more than a convention, the absence of such consent cannot be a legal obstacle to Brexit.

Third, it might be wondered whether the Scotland Act 2016 — enacted in the wake of Scottish independence referendum held in 2014 — makes a difference to all of this. The Act says that “it is recognised that the Parliament of the United Kingdom will not normally legislate with regard to devolved matters without the consent of the Scottish Parliament”. This appears to place the convention mentioned above — that Westminster does not interfere unless the Scottish Parliament consents — on a statutory footing. Does this mean that what was once a political restraint on Westminster has now become a legal constraint, meaning that Scotland can now block Brexit? No.

For one thing, whatever legal force the 2016 Act gives to the convention regarding the need for Scottish consent, it does so in respect of only part of the convention. The Act refers only to Westminster legislation “with regard to devolved matters”. This covers UK legislation that has to do with matters already devolved to Scotland: it is relevant, therefore, if Westminster wishes to make law on a matter that the Scottish Parliament could make (and might already have made) law itself. But the 2016 Act studiously avoids reference to the other aspect of the convention — i.e. the part of the convention that says Scottish consent should be obtained before Westminster adjusts the scope of the Scottish Parliament’s powers. It is that sort of effect that Brexit legislation would presumably have, by removing the requirement that the Scottish Parliament legislates in line with EU law. (Of course, Brexit legislation need not do this — it could leave EU law in place as a restriction on the Scottish Parliament’s powers — but that would be an absurd situation post-Brexit.) The 2016 Act does not therefore give any legal force to the relevant part of the convention concerning the need for Scottish consent to Westminster legislation.

But there is a deeper — and, at the same time, simpler — point concerning the 2016 Act. It is not just that it does not give legal effect to the relevant part of the convention: properly understood, it does not give legal effect to any part of the convention. All it says is that it is recognised that Westminster will not normally legislate on certain matters unless the Scottish Parliament consents. This merely reduces to the UK Parliament — through the medium of the 2016 Act — acknowledging the existence of the political convention concerning the obtaining of consent. It manifestly does not represent a legal constraint on the UK Parliament’s power; nor does it alter the status of the Sewel Convention. In reality, the relevant provision in the 2016 Act is a law that recognises a convention, but, in doing so, the Act does not, through some form of alchemy, turn the convention into a law.

The devolved politics of Brexit are immensely complex and may turn out to be crucially important to what actually happens. And nothing in this post is intended to deny that. But as a matter of law, neither Scotland nor any of the UK’s other constituent nations can stop Brexit from happening.