On 24 September 2019, the Supreme Court (Lady Hale, Lord Reed, Lord Kerr, Lord Wilson, Lord Carnwath, Lord Hodge, Lady Black, Lord Lloyd-Jones, Lady Arden, Lord Kitchin and Lord Sales) declared that the Prime Minister's prorogation of Parliament for five weeks between 9 and 12 September 2019 until 14 October 2019 was unlawful and therefore void and of no effect. Further, the Order in Council proclaiming the prorogation was unlawful and of no effect, and as such Parliament had not been prorogued.
The Supreme Court decision follows a number of decisions in the High Court and Scottish Court of Session regarding the legality of the government's advice to the Queen to prorogue Parliament. Several of these rulings had concluded that the issue was not justiciable on the basis that it was political in nature. However, the Supreme Court has taken the opposite view.
The Supreme Court in its judgment confirmed that the issue was justiciable and that:
The power to prorogue Parliament is a prerogative power, however it is not unlimited and therefore is capable of being subject to judicial scrutiny.
It was concerned with the effect rather than the motive behind the advice to prorogue. If the effect of such a prorogation was that the ability of Parliament to carry out its constitutional functions was frustrated, then it would be unlawful. Here, there was such an effect and there was no reasonable justification given for the extended five-week prorogation, as such, it was unlawful.
It would make a declaration that the advice given to the Crown was unlawful and further, that the Order in Council based on this advice should be quashed and have no effect, meaning that Parliament had not been prorogued.
Clearly, this decision is exceptional and comes at a time of unprecedented political and constitutional uncertainty. However, the Supreme Court has made it clear that such a situation is justiciable by the courts and capable of review and a subsequent declaration of unlawfulness.
(R (Miller) v The Prime Minister, Cherry and others v Advocate General for Scotland  UKSC 41 (24 September 2019)).