The Supreme Court’s decision in Miller v Prime Ministerand Cherry v The Advocate Generalis to be welcomed. It demonstrates a delicate balance between law and politics, affirming the Supreme Court’s role as the guardian of the  UK’s constitution.

The Supreme Court deftly handles issues of justiciability by focusing on the extent as opposed to the exercise of the prerogative power of prorogation. All parties accepted that the courts have the power to determine whether a prerogative power exists and its extent [35]. Prerogative powers are common law powers. The common law determines their scope. The courts are both institutionally and constitutionally suited for this task.

The common law principles the court relied on to limit the prerogative power of prorogation did not require the court to stray into potentially controversial territory. There was no weighing of political choices; no determination of  the correct purposes or length of prorogation; no questioning of Prime Ministerial motives. Instead, the Supreme Court drew on two constitutional principles – parliamentary sovereignty ([41]) and parliamentary accountability ([46]). Both protect democracy. Both empower Parliament, not the courts.

The Supreme Court’s limit on prorogation provides a sound balance between politics and law: ‘a decision to prorogue…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. In such a situation, the court will intervene if the effect is sufficiently serious to justify such an exceptional course.’ ([50]).

The court exercises suitable restraint. Whilst every prorogation harms parliamentary sovereignty and parliamentary accountability, the court will only intervene when the frustration of Parliament is ‘sufficient serious’. Any such intervention is an ‘exceptional course’. Such was the effect of this prorogation. As things currently stand, the UK faces a fundamental change on 31 October. The Prime Minister shut down Parliament for five of the eight weeks left for Parliament to debate Brexit. ‘The democratically elected representatives of the people’ and, not the court, have ‘a right to have a voice’ ([57]) on an issue of such fundamental importance – as the Supreme Court recognised.

The Prime Minister had the opportunity to provide reasonable justification. The Supreme Court found it was ‘impossible…to conclude, on the evidence which has been put before us, that there was any reason – let alone a good reason’ to prorogue Parliament. This suggests that, had reasons been provided, the courts would not have scrutinised them too stringently, respecting the balance between law and politics. But where no reasons are provided, courts cannot speculate on what they might have been.

The decision may be unprecedented. But so was this prorogation. This is no  judicial power-grab, denigrating democracy, and ‘denying 17.4m Brexit’ (Daily Express). It is a carefully reasoned judgment, respectful of the constitutional and institutional limits of the judiciary, which protects ‘the foundations of our constitution’ including ‘representative democracy’ ([55]).

Professor Alison Young, Sir David Williams Chair of Public Law,  University of Cambridge