The judgment on this complex and novel area of constitutional law is all the more powerful in that it was, unexpectedly, a unanimous judgment of all 11 justices. The judgment said that the Prime Minister, Boris Johnson, acted unlawfully when he advised Her Majesty to prorogue Parliament for a period of 5 weeks:
"because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional function without reasonable justification".
The effect of the judgment is that, as a result of this fundamental unlawfulness, everything that followed on from that advice was as if it had never happened. The practical effect was that Parliament had not in law been prorogued and our MPs will be back at work today, occupying the green benches again because it never happened. However the implications of the judgment, even if, as clearly stated by the Court, it was "not about when and on what terms the UK is to leave the EU" will have ramifications for the UK constitution and our understanding of the separation of powers, which underpins our unwritten constitution, for years to come. Commentators have been apologising for using words such as "extraordinary" and "momentous" to describe the judgment but then justify themselves by pointing out how important the judgment is and will be for constitutional relationships. They are correct.
The Court asked itself four questions. Firstly, the question as to whether the lawfulness of the Prime Minister's advice to Her Majesty to prorogue Parliament was a matter which was justiciable and could be reviewed by the Court. Secondly, against what standard was the question of lawfulness to be judged. Thirdly, applying that standard, was the Prime Minister's advice lawful. Fourthly, if the advice was unlawful what remedy should the Court grant.
This is a succinct and clear judgment extending to only 24 pages. It follows on from the conflicting appeals from the High Court in England and Wales and the Inner House of the Court of Session in Scotland. Arguments were put on behalf of the UK Government that the question of prorogation of Parliament and the exercise of the prerogative powers to achieve prorogation was a purely political matter and it was not for the Court to intervene in that political judgment by a Prime Minister. The Court disagreed with that argument. It said:
"…the courts have exercised a supervisory jurisdiction over the decisions of the executive for centuries"
The justices also said:
"The fact that the minister is politically accountable to Parliament does not mean that he is therefore immune from political accountability to the courts".
Having determined what standards should be applied and looking at constitutional principles and case law, they stated:
"For the purposes of the present case, therefore, the relevant limit upon the power to prorogue can be expressed in this way: that a decision to prorogue Parliament (or to advise the monarch to prorogue Parliament) 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 such a situation, the court will intervene if the effect is sufficiently serious to justify such an exceptional course……
It is impossible for us to conclude, on the evidence which has been put before us, that there was any reason - let alone a good reason - to advise Her Majesty to prorogue Parliament for 5 weeks, from 9th or 12th September until 14th October. We cannot speculate, in the absence of further evidence, upon what such reasons might have been. It follows that the decision was unlawful."