To what degree did the Supreme Court limit Boris Johnson's ability to prorogue?

Question

From the UK Supreme Court's summary of today's hearing on prorogation of Parliament:

The next and final question, therefore, is what the legal effect of that finding is and therefore what remedies the Court should grant. The Court can certainly declare that the advice was unlawful. The Inner House went further and declared that any prorogation resulting from it was null and of no effect. The Government argues that the Inner House could not do that because the prorogation was a “proceeding in Parliament” which, under the Bill of Rights of 1688 cannot be impugned or questioned in any court. But it is quite clear that the prorogation is not a proceeding in Parliament. It takes place in the House of Lords chamber in the presence of members of both Houses, but it is not their decision. It is something which has been imposed upon them from outside. It is not something on which members can speak or vote. It is not the core or essential business of Parliament which the Bill of Rights protects. Quite the reverse: it brings that core or essential business to an end.

This Court has already 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. This is the unanimous judgment of all 11 Justices.

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. It is not clear to us that any step is needed from the Prime Minister, but if it is, the court is pleased that his counsel have told the court that he will take all necessary steps to comply with the terms of any declaration made by this court.

If I am understanding this correctly, the court not only ruled in favor of the UK Parliament, but also stripped the UK government of its ability of proroguing Parliament against its will in the future -- nullifying the PM's claim that they'd consider proroguing again in the process. Am I misreading something, or was this indeed a giant slap in the face?

Answer 1

It is far too early to tell to what degree the Government's options have been constrained. A key part of the advice is paragraph 50;

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.

The prorogation would be lawful with a reasonable justification. No such justification was given this time. Whether any justification a government may provide in future is reasonable is pure speculation at this point.

Later in the judgement, we see that;

59. The unchallenged evidence of Sir John Major is clear. The work on the Queen’s Speech varies according to the size of the programme. But a typical time is four to six days.

It seems likely that a prorogation of longer than this period would need a very clear justification to avoid a legal challenge at a minimum.

Answer 2

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.