The Government’s decision to use the prorogation power for questionable political purposes was a provocation to the courts. Unfortunately, the Supreme Court’s understandable discomfort with the Government’s move led it to search for, and ultimately create, a legal remedy for a political problem. In doing so it has imposed an entirely novel and potentially open-ended legal limitation upon one of the clearest and most unequivocal powers of the Crown. This is unprecedented not only in the United Kingdom but within any Westminster system of government where the prerogative of prorogation, (one of the ‘prerogatives of sovereignty’) has hitherto been treated as beyond the reach of the courts.
The Court calls upon two principles to defend its radical innovation; the application of each is open to scrutiny. The first is, ironically, parliamentary sovereignty. This principle, however, is not concerned with the sitting of Parliament but with its legislative authority. When courts use parliamentary sovereignty to justify the constraint of a lawful competence they tend naturally to do so in the context of statute; citing Parliament’s intention to limit a particular power. Not only was the Supreme Court unable to do so in relation to prorogation, Parliament unequivocally eschewed any legal circumscription of this prerogative power in the Fixed-term Parliaments Act 2011, s6(1): ‘This Act does not affect Her Majesty’s power to prorogue Parliament’. In passing the Northern Ireland (Executive Formation etc.) Act 2019, Parliament provided for the possibility of recall during a prorogation period by way of a Royal Proclamation (s.3(4)) but did not seek to delimit the scope of the power itself. In light of these provisions, a justification for the Court’s decision founded in Parliament’s legal supremacy is hard to find.
The second is the ‘fundamental principle’ of ‘Parliamentary accountability’. Accountability is indeed central to our system of government but it is an amorphous constitutional concept with no legal source in prerogative, statute or common law. There are of course conventions of ministerial responsibility to Parliament but it is long settled that these are not judicially enforceable. Somehow in Miller/Cherry  the elastic principle of accountability was ascribed with sufficient normative authority to justify placing legal limits on the prerogative of prorogation, despite the specific exclusion of this prerogative from legal controls by Parliament itself. The Court also goes further, seemingly using this principle to put the legal onus on the Crown to justify its use of the power:
‘It is impossible for us to conclude, on the evidence which has been put before us, that there was any reason – let alone a good reason – to advise her Majesty to prorogue Parliament for five weeks…We cannot speculate, in the absence of further evidence, upon what such reasons might have been.’ 
This suggests that the principle of parliamentary accountability has acquired the authority of a new precautionary principle in relation to prorogation; with the Crown, if challenged, now required to offer acceptable political reasons to the courts for any decision to prorogue.
“The UK Parliament is not seeking to convert the Sewel Convention into a rule which can be interpreted, let alone enforced, by the courts… We would have expected the UK Parliament to have used other words if it were seeking to convert a convention into a legal rule justiciable by the courts.” (‘Miller 1’, para 148).
Indeed; similarly, we would expect Parliament to have used other words in the FTPA if it were seeking to curb the sovereign power of prorogation. It is very hard so see how, in the absence of any statutory authority, and in light of the FTPA and recent Northern Ireland (Executive Formation etc.) Act, the fluid principle of accountability should be ascribed sufficient clarity and force to quash such a fundamental prerogative power.
How did the Court justify arming a constitutional principle with such legal salience? It took the view that: ‘The effect [of the decision to prorogue] upon the fundamentals of our democracy was extreme.’ There will be different views about the accuracy of this assessment. It is however a politicalview; a political view that led to the identification first of a constitutional principle and then the creation of a legal rule that served to normativise this principle even to the point of constraining a prerogative of sovereignty. The Court’s concern with the Government’s failure to articulate good reasons for prorogation was understandable, but one danger in articulating principles which hitherto have had no legal status and equipping them with such potent normative force is that similar legal claims can be made in relation to other ‘principles’ in difficult political situations. The risk is that the legal system will thereby be opened to further weaponisation by both sides of the Brexit debate. This is not good for the courts and certainly not good for the constitution. The Prime Minister’s move to seek prorogation was certainly constitutionally questionable, but Parliament is the forum within which such behaviour should be challenged.
Stephen Tierney is Professor of Constitutional Theory, University of Edinburgh