On 24 September 2019, the UK Supreme Court, in a unanimous judgment, held that he advice given by the Prime Minister to Her Majesty the Queen on 27th or 28th August, that Parliament should be prorogued from a date between 9th and 12th September until 14th October, was unlawful. In a landmark case, the court held that it had the authority to review whether the Prime Minister’s advice to Her Majesty was lawful. It also held that a decision to prorogue (or advise the monarch to prorogue) will be unlawful if the prorogation has the effect of frustrating or preventing, without reasonable justification, the ability of Parliament to carry out its constitutional functions as a legislature and as the body responsible for the supervision of the executive. In judging any justification which might be put forward, the court said it must of course be sensitive to the responsibilities and experience of the Prime Minister and proceed with appropriate caution. If the prorogation does have that effect, without reasonable justification, the court said there is no need for it to consider whether the Prime Minister’s motive or purpose was unlawful. The court said that this prorogation did have the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions without reasonable justification.
This Court concluded that the Prime Minister’s advice to Her Majesty was unlawful, void and of no effect. This means that the Order in Council to which it led was also unlawful, void and of no effect and should be quashed. This means that when the Royal Commissioners walked into the House of Lords it was as if they walked in with a blank sheet of paper. The prorogation was also void and of no effect. Parliament has not been prorogued. Final the court said:
It is for Parliament, and in particular the Speaker and the Lord Speaker to decide what to do next. Unless there is some Parliamentary rule of which we are unaware, they can take immediate steps to enable each House to meet as soon as possible.
[R (on the application of Miller) (Appellant) v The Prime Minister (Respondent) Cherry and others (Respondents) v Advocate General for Scotland (Appellant) (Scotland) On appeals from:  EWHC 2381 (QB) and  CSIH 49]