The Supreme Court judgment on the legality of a Scottish independence referendum can be broken down into three elements. Two are unsurprising but the third is very problematic.

The first decision was to accept the case at all. The UK Government had argued that the issue was hypothetical and premature and that a judgment should have been made only on a complete Bill. This would have put pressure on the Presiding Officer and the Lord Advocate and caused confusion and a potential politicisation of those roles, while the Supreme Court would have eventually had to rule in any case.

The second decision was that a referendum, albeit consultative, would affect the union. It might not have legal effects but it would have practical effects in the form of political consequences. This is an unsurprising interpretation of the Scotland Act.

The third decision was to reject the SNP’s argument that Scotland has the right of self-determination in international law. International laws is not at all clear on this matter but there is a body of opinion that secession is only permissible in the case of colonies or where there has been manifest oppression of a people. This does not apply in the case of Scotland. In making this argument, the Supreme Court invoked the judgment of the Supreme Court of Canada when it was asked by the Government of Canada whether Quebec had the right to secede. As the UK Court notes, the Canadian Court ruled that Quebec did not have that right. What the Court did not mention is the other part of the Canadian judgment, that if Quebec or any other province did vote for independence by a clear majority on a clear question, the Government of Canada would be bound to negotiate. This was because the Canadian Constitution must be read in the light of democratic principles. The UK Supreme Court, on the contrary, argued that, precisely because a referendum would be an expression of the democratic will of the Scottish people, it would have political consequences and therefore be illegal.

 A clear outcome, whichever way the question was answered, would possess the authority, in a constitution and political culture founded upon democracy, of a democratic expression of the view of the Scottish electorate. The clear expression of its wish either to remain within the United Kingdom or to pursue secession would strengthen or weaken the democratic legitimacy of the Union, depending on which view prevailed, and support or undermine the democratic credentials of the independence movement (para 81)

Denying a referendum on these grounds comes close to denying that Scotland has a right to self-determination, which goes further than either this or previous UK Governments have gone. Even Margaret Thatcher wrote that:

As a nation, they (the Scots) have an undoubted right to national self-determination; thus far they have exercised that right by joining and remaining in the Union. Should they determine on independence no English party or politician would stand in their way, however much we might regret their departure (The Downing Street Years, p.624).

It would perhaps have been unrealistic to expect the UK Supreme Court judges to act otherwise than they did. They have consistently adhered to the letter of the law rather than engaging in broader constitutional reasoning. Yet this risks undermining the importance of the conventions and understandings on which our largely unwritten constitution depends. In the Miller judgment a few years ago the Court were asked whether the Sewel Convention, under which Westminster only ‘normally’ legislates on devolved matters with the consent of the devolved legislatures, applied to the triggering of the Brexit process. Instead of saying that the situation was not normal or that the matter was reserved, the Court took the opportunity to double down on parliamentary sovereignty and supremacy and effectively declared that the Convention had no constitutional standing at all.

Sometimes legal clarity can get in the way of the margin for interpretation that our constitution gives for politicians to negotiate ways of out deadlocks. The way is now open for the UK Government to say that there is no time or way for Scotland to exercise its acknowledged right of self-determination.

Michael Keating is Emeritus Professor of Politics at the University of Aberdeen