While many lawyers and legal commentators were speculating that the government had lost the Supreme Court case over prime minister Boris Johnson’s decision to prorogue (suspend) parliament, no one had predicted the scale of his defeat. In the unanimous judgment in R (Miller) v Prime Minister; Cherry and others v Advocate General for Scotland [2019] UKSC 41, 24 September 2019, the 11 judges ruled that the prime minister’s decision was ‘null and of no effect’ (para 69) and so parliament had not been prorogued.

The principle of the supremacy of parliament is at the core of the judgment. It sets out the constitutional position clearly, observing that the executive branch of government is not directly elected but ‘exists because it has the confidence of the House of Commons’ and ‘has no democratic legitimacy other than that’ (para 55). The court found that all the parties to the case accepted that the courts have the authority to rule on ‘the existence of a prerogative power’ (para 36), and, with regard to the prime minister’s power to prorogue parliament, if this was not limited, it would be ‘incompatible with the legal principle of parliamentary sovereignty’ (para 42).

In her oral summary of the judgment, Lady Hale, president of the Supreme Court, did not pull any punches in explaining the court’s reasoning in overturning Johnson’s decision. She said that ‘[n]o justification for taking action with such an extreme effect has been put before the court’, and that the court was ‘bound to conclude, therefore, that the decision to advise Her Majesty to prorogue parliament was unlawful because it had the effect of frustrating or preventing the ability of parliament to carry out its constitutional functions without reasonable justification’.

Speaking to the BBC in response to the judgment, Johnson said: ‘Obviously, this is a verdict we will respect,’ but he added that he strongly disagreed with what the court had ruled. He also left open the possibility of exercising his power to prorogue parliament again in the near future, remarking that there was ‘a good case for getting on with a Queen’s Speech’.

Dominic Grieve QC MP, the former attorney general, who recently lost the Conservative whip, told the BBC that the judgment had brought about a ‘constitutional change’ as the ‘idea that you can play fast and loose with parliament, just to nudge it to one side when it’s convenient for the executive to do so, is dead and buried for good’. He said Johnson’s ‘behaviour was fundamentally wrong’ and explained he believes that, due to the Supreme Court’s decision, if he were to try to prorogue parliament again in the coming days, there would be an ‘immediate order’ to say it was illegal.

It remains to be seen what the political fallout from the case will be. The leader of the opposition, Jeremy Corbyn, and others are calling for Johnson’s resignation. The consensus among lawyers is more clear-cut: this judgment is one of the most constitutionally significant in years.