In Cherry/Miller, the Supreme Court chose to change the law by creating a new legal limitation on the prerogative to prorogue Parliament.  Whether one thinks the Court was within its rights to change the law, the judgment was, from a practical point of view, clearly lawmaking with retrospective effect, even if the fiction that the common law has always existed allowed it to be presented as a declaration of the current law.

Neither Parliament nor the courts should change the law retrospectively without good reason and sensitivity to the injustice of stigmatising conduct as unlawful when that is not what it would have been reasonable to expect when it took place   The Court did not appear to recognise or attempt to justify the incompatibility of its decision with the rule of law and its core value of securing predictability in the law.

The judgment was uncompromising in its tone and outcome: a total rejection of the Government’s arguments (and the careful reasoning of judges at earlier stages).  A long career in the production of legislation taught me that reliance on authority alone to make new law seldom ends well, even when the authority in question is as uncontroversial as Parliamentary Sovereignty, and the legislation is prospective. Those subject to new law do always need to be persuaded to accept its legitimacy and its fairness. Satisfying that need is an important function of Parliament’s legislative procedures. An uncompromising assertion of authority, dismissing out of hand the concerns and arguments of those whom you expect to submit themselves to your authority and to accept its legitimacy, is the wrong way to go about making sure they do.

So, the immediate effect of the Court’s disregard for that principle – making new law and trenchantly and retrospectively enforcing it – was an inevitable and entirely predictable elevation in the temperature of the political conflict: as the different sides responded to the decision with, respectively, triumphant condemnation of the Prime Minister and his advisers and defiant denial of the correctness of the decision.

When you have a court departing from settled law to tell you who is right and who is wrong in suitably uncompromising terms – dealing out political damage to one side and political advantage to the other – the incentive to conciliate your political opponents disappears. Hopefully, this effect will be only temporary, but the Court will deserve no credit if it is.

Not only was the judgment polarising, it laid down principles that are so vague that they fail the test of clarity and predictability that, in all other circumstances, the rule of law rightly requires of lawmakers. The crucial elements of the unlawfulness are all left unexplained, or said to be obvious, or they are demonstrated by speculation about what might have happened in other circumstances, rather than by an analysis of what actually did happen.

One obvious effect of the judgment, since the resumption of Parliament, has been the struggle of Parliament to find useful things to do with the time that has been restored to it, not least because the invalidated prorogation had not in fact frustrated what it felt it needed to do.

The judgment fails the very test it had itself applied to the prorogation. It fails to provide a reason for a decision (to subject the political process of prorogation to judicial regulation) the effects of which, even apart from the polarisation, were damaging to the UK’s constitutional arrangements.

The judgment concedes that “the actual task of governing is for the executive and not for Parliament or the courts”.  However, the judgment implicitly endorses and encourages parliamentary conduct that involves and relies upon highly questionable (but of course, under Article IX of the Bill of Rights 1688, rightly unchallengeable) changes in parliamentary procedure, some overturning or disregarding established rules and conventions.

Those changes have deprived the Government of the advantages in the management of parliamentary business that have conventionally been conferred on it for so long as the House of Commons does not formally withdraw its confidence. Those advantages are vitally important because they are all that secures coherent and coordinated national governance where an election has produced a result requiring a period of minority government.

The quite mistaken conception of Parliament’s role that these changes have presupposed has produced the paralysis in Parliament that is now undermining responsible parliamentary government. The Government is kept in power – so that it must take responsibility for the actions that others force upon it – while those others themselves evade accountability for the decisions, as well as the judgement of the electorate.

The Court has removed the equality of arms that might have allowed the Government to use prorogation proportionately to mitigate the effect of those undesirable changes.  This is hugely damaging. Moreover, it recklessly creates a risk of associating the Court, however undeservedly, with the perceived loss of impartiality of the Speaker who has facilitated those changes.

When the courts make new law, they make choices that are not forced on them by existing law. I can find no reason in the judgment, let alone a good one, for choosing to strengthen – as the judgment does – the factors that maintain the current deadlock in parliamentary politics.

Sir Stephen Laws KCB, QC (Hon) is Senior Research Fellow, Policy Exchange and former First Parliamentary Counsel