On 17 September 2019, the UK Supreme Court will be required to choose between, or somehow reconcile, different decisions of the English High Court and the Scottish Court of Session on the lawfulness of Prime Minister Johnson's decision to prorogue Parliament.
On 11 September 2019, The High Court issued its full judgment in Gina Miller v Prime Minster  EWHC 2381 (QB). The English court found that the Prime Minister's decision to prorogue Parliament from 10 September to 14 October was not subject to legal challenge (it was not "justiciable") because it was "inherently political in nature". The English court said that the "the essential characteristic of a “political” issue is the absence of judicial or legal standards by which to assess the legality of the Executive’s decision or action". It would be impossible for the court to make a legal assessment of whether the duration of the prorogation was excessive by reference to any measure. There is no legal measure of the length of time between Parliamentary sessions. There is not even a constitutional convention which governs the matter. Consequently, the High Court concluded that "the decision of the Prime Minister to advise Her Majesty the Queen to prorogue Parliament is not justiciable in Her Majesty’s courts".
Also on 11 September 2019, the Scottish Court of Session found, on appeal, that "the Prime Minister’s advice to HM the Queen and the prorogation which followed thereon was unlawful and is thus null and of no effect". In reaching their unanimous decision, the Scottish judges found that "the particular prorogation that had occurred, as a tactic to frustrate Parliament, could legitimately be established as unlawful. This was an egregious case of a clear failure to comply with generally accepted standards of behaviour of public authorities. It was to be inferred that the principal reasons for the prorogation were to prevent or impede Parliament holding the executive to account and legislating with regard to Brexit, and to allow the executive to pursue a policy of a no deal Brexit without further Parliamentary interference". In effect, the Scottish court found that the government's ulterior motives (to "stymying Parliament") brought the decision within the court's power to review and declare it unlawful.
Both judgments are due to be considered by the UK Supreme Court on 17 September. In practice, it is likely that the Supreme Court will have to choose between the different approaches stemming from the distinct legal traditions of the English and Scottish courts. It would be odd and probably unworkable to find that the outcome on a fundamental question of UK constitutional law differs, so that a government decision might be lawful in England but simultaneously unlawful in Scotland. Given that the Scottish court's judgment rests on inferences concerning the government's motivation and intentions, it might be open to the Supreme Court to characterise those motivations as "inherently political" and to find that, on balance, it is not for the UK Supreme Court to intervene. That would not dilute the force or impact of a finding by Scotland's most senior and respected judges that the government's aim was to avoid Parliamentary scrutiny, or detract from Lord Drummond Young's observation:
"It was incumbent on the UK Government to show a valid reason for the prorogation, having regard to the fundamental constitutional importance of parliamentary scrutiny of executive action. The circumstances, particularly the length of the prorogation, showed that the purpose was to prevent such scrutiny. The documents provided showed no other explanation for this. The only inference that could be drawn was that the UK Government and the Prime Minister wished to restrict Parliament".