R (Miller) v Prime Minister; Cherry v Advocate General for Scotland [2019] UKSC 41

On 27 or 28 August 2019, the Prime Minister advised the Queen to prorogue Parliament on a date between 9 and 12 September until 14 October. Parliament was prorogued on 9 September. The suspension covered five of the eight weeks remaining before the United Kingdom's expected departure from the European Union on 31 October. Gina Miller challenged the advice in England, while Joanna Cherry and others brought proceedings in Scotland. The Divisional Court regarded the issue as non-justiciable, but the Inner House held the advice unlawful. The Supreme Court convened all eleven justices to resolve the appeals and determine the legal consequences.

Could the courts review the prorogation advice, and did its effect on Parliament's legislative and supervisory functions make the advice unlawful?

The Supreme Court unanimously held that the issue was justiciable and the advice unlawful. Courts may determine the legal limits of a prerogative power, drawing on enforceable constitutional principles. A prorogation is unlawful if it frustrates or prevents, without reasonable justification, Parliament's ability to legislate and supervise the executive, where the effect is sufficiently serious. A five-week suspension at this exceptional moment had an extreme effect on those functions. The Government supplied no reason, let alone a good reason, for that duration. The advice, the resulting Order in Council and the purported prorogation were therefore null. Prorogation was not protected as a proceeding in Parliament by article 9 of the Bill of Rights 1689, so Parliament had not been prorogued.

The prerogative power to prorogue Parliament is limited by parliamentary sovereignty and executive accountability. A decision is unlawful where, without reasonable justification, it has the effect of frustrating or preventing Parliament from carrying out its constitutional functions, and that effect is sufficiently serious to warrant judicial intervention.

The judgment is a major modern authority on judicial control of prerogative power and the legal force of constitutional principles in an uncodified constitution. It did not decide whether the Prime Minister acted from an improper motive; unlawfulness followed from the prorogation's effect and lack of justification. The remedy also confirmed that courts can declare purported prerogative action legally void without infringing parliamentary privilege. The case should be distinguished from judicial supervision of Parliament's internal proceedings, because the court held that prorogation itself was imposed from outside Parliament.