In the opening paragraph of the Supreme Court's decision on the lawfulness of the Prime Minister's decision to ask the Queen to prorogue Parliament between 9th - 12th September 2019 to 14 October 2019 Lady Hale acknowledges that this is a one off decision, arising from circumstances never seen before and unlikely to be seen again.

The decision does not deal with the lawfulness or not of leaving the European Union. It deals only with the lawfulness of the request made to Her Majesty to prorogue. 

The first question for the Supreme Court was whether they had jurisdiction to consider this matter, or whether, as had been held at the High Court of England and Wales, this was a matter of Parliamentary business and was not justiciable in a court of law. On this point, the Supreme Court agreed with decision made on 11 September 2019 by the Inner House of the Court of Session in Scotland, namely that the matter was justiciable and could be decided by the Supreme Court.

The Supreme Court then went on to consider whether the decision to advise the Queen to prorogue was unlawful. The Court held that it would be unlawful if it has the effect of frustrating or preventing, without reasonable justification, the ability of Parliament to carry out its constitutional functions.

It was then for the Court to consider whether the advice to Her Majesty had frustrated or prevented Parliament to carry out its functions and whether there was reasonable justification for this. In the judgement, the panel of 11 Justices unanimously found against the Prime Minister and concluded that the advice to prorogue was unlawful. 

Weight was attached to the timing and impact of the prorogation, namely that it dramatically reduced Parliamentary time to debate the process for the withdrawal of the UK from the European Union on 31 October 2019.

"This prolonged suspension of Parliamentary democracy took place in quite exceptional circumstances: the fundamental change which was due to take place in the Constitution of the United Kingdom on 31st October. Parliament, and in particular the House of Commons as the elected representatives of the people, has a right to a voice in how that change comes about. The effect upon the fundamentals of our democracy was extreme. 

No justification for taking action with such an extreme effect has been put before the court.....

...The Court is 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."

The judgement concludes that the prorogation is null and void and has no effect: "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". It is as though the prorogation of Parliament did not happen. 

It is for the Speakers of the two Houses to decide what happens next. It would be possible for Parliament to sit immediately and to continue with Parliamentary business, albeit with some bleary eyed Labour MPs who will have come straight from their Labour Party annual conference.