[2][3] Comparable contemporary events in other Commonwealth countries that were highly controversial include the 2008 prorogation of the Parliament of Canada, which prevented the Prime Minister of Canada, Stephen Harper, from losing a vote of no confidence;[4] the 2018 Sri Lankan constitutional crisis, in which the Supreme Court unanimously ruled that President Maithripala Sirisena's attempt to dissolve Parliament was unlawful and void; and "The Dismissal", in which the Prime Minister of Australia, Gough Whitlam, was dismissed by the Governor-General, John Kerr, and Whitlam's successor, Malcolm Fraser, requested the double dissolution of Parliament in advance of a federal election before the Labor-controlled House of Representatives could reinstate Whitlam.

[4] May was succeeded in the following party leadership election by Boris Johnson,[4] whose campaign team had floated the possibility of prorogation to force a no-deal Brexit despite Parliament overwhelmingly rejecting the proposition.

[9] In late July, the newly appointed Leader of the House of Commons, Jacob Rees-Mogg, said the government viewed prorogation for political purposes as an "archaic mechanism" which would not be used.

[17] Victims' rights activist Raymond McCord made an application at the High Court of Northern Ireland in Belfast which alleged breaches of the Good Friday Agreement.

[23] During the Court of Session hearings on 3 September, the court heard evidence that Johnson had approved negotiations with the Palace on 15 August 2019, by way of signing a handwritten note to his special adviser Nikki da Costa and Dominic Cummings, and made comments about the short sitting of Parliament in September being a "rigmarole" to show MPs were "earning their crust".

Due to the significance of the case, the maximum eleven of the twelve Supreme Court justices sat to hear the appeal,[28] with Lord Briggs not sitting to ensure an odd number of judges.

The court allowed six interveners to make representations over the course of the hearing: Raymond McCord, whose case was not heard alongside Miller and Cherry; the Lord Advocate for Scotland, James Wolffe; the Counsel General for Wales, Jeremy Miles; former Prime Minister John Major; the Shadow Attorney General, Shami Chakrabarti; and The Public Law Project.

Lord Pannick, who responded on Miller's behalf, argued that there was "strong evidence" that the purpose of prorogation was to prevent MPs from "frustrating" the government's Brexit plans, and that the court was entitled and obligated to deliver verdicts on the rule of law.

O'Neill, who represented the Cherry litigants, argued that the decision to prorogue was "taken in bad faith" and "for an improper purpose" and that the Court of Session opinion offered an outsider perspective "400 miles from Westminster" to that effect.

[31] The final day of the hearing saw interventions from other interested parties: Major's former Solicitor General, Lord Garnier, argued prorogation was "motivated by a desire to prevent Parliament interfering with the Prime Minister's policies during that period"; the Scottish Government, who were represented by the Lord Advocate, argued prorogation had a "profoundly intrusive effect" on Parliament; McCord's advocate Ronan Lavery argued prorogation was designed to "run down the clock" to force a no-deal Brexit, which would in turn result in controls on the border with Ireland; and in a written submission, the Shadow Attorney General, Shami Chakrabarti, said that if the power to prorogue was unchecked, Parliament would be "deprived" of the ability to "perform its constitutional function".

The court utilised a three-prong test in determining the case:[33] The judgment is significant for its treatment of the principle of justiciability, its interpretation of elements of the British constitution, and its potential implications for the separation of powers.

[35] By contrast, Richard Ekins, an associate professor of law at the University of Oxford, called it "a startling judgment" that was "badly mistaken" and that the court showed "a clear loss of faith in the political process" when it ruled in an area that he and many other lawyers previously thought it did not have jurisdiction to do so.

[36] In the same vein, John Finnis, professor emeritus of law and legal philosophy at the University of Oxford, considered that the Supreme Court had "forayed" into politics, calling the judgment "a historic mistake" and "a misuse of judicial power".

"[40] Ali reiterated an argument he made before the judgment in the Oxford University Political Blog that in lieu of dissolution and prorogation, future prime ministers may ask the sovereign to refuse royal assent to any bill until the House of Commons agreed to call an early general election, which he argued would likely cause far greater public outrage than prorogation.

"[41] Robert Blackburn, a Professor of Constitutional Law at King's College London, argued in a different submission to the same committee that repeal or reform of the Fixed-term Parliaments Act 2011 would potentially provide a convenient opportunity for prorogation to become subject to a vote in both Houses of Parliament on a motion moved by the government – but did not consider the potential impact on the prerogative power of royal assent.

Johnson said that he "strongly disagreed" with the ruling, but the government would "respect the judicial process" and not prevent Parliament from meeting; he also stated his preference for a new parliamentary session and Queen's Speech after a lawful prorogation.

He defended the advice he gave to Johnson on the constitutionality of the prorogation as being "in good faith", and that other senior legal professionals and lower courts agreed with the government's arguments.