The court put stringent limits that prevent a PM to abuse the power
If the judgement's pdf is to go by, oversight on whether prorogation is justified or not seems to now be firmly in the Court's hands.
This is very new. Basically, the PM can no longer prorogue on a whim, and the court took and reserved the right to decide -- complete with guidance for lower courts -- whether justifications being put forward to prorogue parliament in the future are legit or not.
Two key criteria were put forward.
The most important one is whether it gets in the way of Parliament's ability to do its job. Namely, to debate laws and scrutinize government. Based on that criteria, there's basically zero chance that Johnson will try proroguing again for more than a few days -- he will fail if he does, on the basis that he's doing so at a critical moment that requires parliamentary scrutiny.
The other, less important one, is how short or long it is, and the court makes clear that a few days is the expected reasonable length without extraordinary justification. Johnson might get away with a few days prorogation that includes the coming WE (so Tories can have their conference). But not much more.
That being said, the most interesting aspect of this ruling has nothing to do with the matter that triggered the ruling. In declaring the decision to prorogue parliament null and void, the court basically upheld the sovereignty of Parliament, plain and simple. Had it sided with government, the latter would have been able to prorogue on a whim when Parliament wasn't on its side. For this reason, this judgement is likely to end up in history books IMHO.
Articles 50-51 are of particular interest (emphasis mine):
50. 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.
51. That standard is one that can be applied in practice. The extent to which prorogation frustrates or prevents Parliament’s ability to perform its legislative functions and its supervision of the executive is a question of fact which presents no greater difficulty than many other questions of fact which are routinely decided by the courts. The court then has to decide whether the Prime Minister’s explanation for advising that Parliament should be prorogued is a reasonable justification for a prorogation having those effects. The Prime Minister’s wish to end one session of Parliament and to begin another will normally be enough in itself to justify the short period of prorogation which has been normal in modern practice. It could only be in unusual circumstances that any further justification might be necessary. Even in such a case, when considering the justification put forward, the court would have to bear in mind that the decision whether to advise the monarch to prorogue Parliament falls within the area of responsibility of the Prime Minister, and that it may in some circumstances involve a range of considerations, including matters of political judgment. The court would therefore have to consider any justification that might be advanced with sensitivity to the responsibilities and experience of the Prime Minister, and with a corresponding degree of caution. Nevertheless, it is the court’s responsibility to determine whether the Prime Minster has remained within the legal limits of the power. If not, the final question will be whether the consequences are sufficiently serious to call for the court’s intervention.
