The prorogation of Parliament or the advice on which such prorogation was based has not, historically, been thought to have been justiciable. The Supreme Court has decided that it is justiciable but determines that question not separately or as a preliminary issue but in combination with a determination on two other issues, namely “where a legal limit lies in relation to the power to prorogue Parliament, and whether the Prime Minister’s advice trespassed beyond that limit” ().
The Court’s combined treatment of these questions, the question of justiciability, the existence and location of the legal limits on prorogation, and whether or not these have been exceeded gives rise to some queries which the judgment itself cannot answer, or, at least, not answer in a satisfactory way.
The Court thinks that Parliamentary sovereignty (in the expanded sense used in para  of the judgment) would be undermined if “there was no legal limit upon the power to prorogue Parliament (subject to a few exceptional circumstances in which, under statute, Parliament can meet while it stands prorogued)” ().
Setting aside both these ‘few exceptional circumstances’ and the practical limitations on the power to prorogue (to which the Court does not give much weight) the Court goes from seeing “scant reassurance” in the latter (at ) to a conclusion for which the reader is, as respects its logic, ill prepared: “It must follow, as a concomitant of Parliamentary sovereignty, that the power to prorogue cannot be unlimited” ().
Setting aside the logical strength or weakness of this conclusion by reference to what has preceded it in the judgment, it is odd that the Court follows it by instancing seven statutes which impose statutory requirements about the sitting of Parliament. Of these statutes the Court says, “Their existence confirms the necessity of a legal limit on the power to prorogue” ().
It might be thought more accurate to say that these statutes show (1) that there are limits to the power to prorogue, (2) that these limits were set by Parliament itself, and (3) that Parliament thought that these limits were necessary or, at least, desirable.
But if there are limits, as the Court acknowledges, to the power to prorogue and these have been imposed by Parliament itself, why, it might be asked does the Court consider that it is legitimate for the Court to create new limits?
In deciding what these new limits should be the Court brings into play what it has identified as a second constitutional principle, that of Parliamentary accountability. I have not spotted in the judgment any discussion of the statutorily prescribed consequences if the House of Commons passes a motion of no confidence in Her Majesty’s Government and it cannot be said so long as this powerful provision (section 2(3) of the Fixed-term Parliaments Act 2011) exists that parliamentary accountability of the most significant and weighty character is not present. And this species of parliamentary accountability was deployable during the first week of September.
This is not, in the view of the Supreme Court sufficient because it decides to create its own limitation, as follows: according to the Supreme Court “a decision to prorogue Parliament (or to advise the monarch to prorogue Parliament) will be unlawful if the prorogation has the effect of frustrating or preventing, without reasonable justification, the ability of Parliament to carry out its constitutional functions as a legislature and as the body responsible for the supervision of the executive.”
Having created its own limitation, the Court then works backwards and, based on the limitation that it has just created, concludes that the (new) lawful extent of exercise of the prerogative power to prorogue “by definition … concerns the extent of the power to prorogue, and is therefore justiciable.” And the limit transgressed.
Is it too much to suggest that the Court’s discovery of this new limitation (and that it has been breached) reminds one a little of Captain Renault’s shocked discovery of gambling in Rick’s Café Americain?
John Larkin QC is Attorney General for Northern Ireland