The Supreme Court has today rejected the Government’s appeal from the High Court judgment by a majority of eight justices to three. The decision means that a new Act of Parliament will now be required before the Government may lawfully trigger Article 50. However, the Court has also unanimously dismissed the devolution challenges, which argued that the consent of the devolved legislatures in Scotland, Wales and Northern Ireland was a constitutional precondition to Brexit.

The judgment is obviously important, but perhaps less important than once assumed. The litigation was launched immediately after the referendum. While it was framed as an attempt to vindicate parliamentary sovereignty, the point of the litigation has always seemed to be to frustrate implementation of the referendum result, by delaying the process and giving MPs and Lords a chance to obstruct it.

The political subtext of the litigation was not a reason for the courts to turn the claim away. The litigation raised an arguable point of law, but a point of law that most constitutional lawyers initially thought was ambitious at best or hopeless at worst. It was a surprise when the claim succeeded in the High Court. That judgment has since been subjected to robust legal and scholarly criticism, as well as giving rise to unwarranted personal attacks on the judges.

The Supreme Court should have allowed the Government’s appeal. The High Court mishandled the relevant law, partly because the Government did not argue its case as well as it could have. But the Supreme Court has much less excuse for going wrong. It had the time and opportunity to consider the flaws in the High Court’s reasoning and the advantage of much better legal argument from the Government.

The central question in the appeal was how to understand the European Communities Act 1972. Like the High Court, the majority in the Supreme Court misinterpreted the 1972 Act, concluding that Parliament in 1972 did not envisage UK membership of the EU (then the EEC) being set aside by the executive alone, without specific parliamentary authorisation.

An important premise for this reading of the Act was the majority’s view that the Act made European law in some way a direct source of law in the UK, the bringing to an end of which would be a major constitutional change requiring express legislative authorisation.

Here the dissent is more persuasive. Lord Reed, with whom Lord Carnwath and Lord Hughes agreed (although each wrote a short judgement of their own), pointed out that EU law has effect only on the terms provided for in the 1972 Act, as the Supreme Court has made clear several times before and as the European Union Act 2011 also reiterates.

The majority conceded that the content of EU law in force in the UK changes from time to time, without any need for a new Act of Parliament. That is, the prerogative is often lawfully used, in engaging with the EU institutions, to vary the content of resulting legal rights in the UK. Variation in this way was in accord with the terms of the 1972 Act, which refer to obligations under the Treaties as they stand “from time to time”. However, the majority insisted that withdrawal from the Treaties was different in kind and that the Act impliedly forbade this further step.

Lord Reed again shows how unreal this is. In a painstaking analysis of the context of the 1972 Act and the way in which the UK came to be a member of the EEC – by way of the exercise of the prerogative to enter into the Treaties – he makes clear that Parliament did not at all intend to limit the Crown’s continuing powers in relation to foreign affairs. Rather, it took them for granted. The majority draws an implication from the Act, Lord Reed concludes, without foundation.

No one in the litigation questions parliamentary sovereignty and indeed one silver lining of the majority’s decision is its clear affirmation of this fundamental principle. The dissenting judges, agreeing with the Government and with many eminent constitutional lawyers, conclude that the use of the prerogative in no way contradicts or displaces any statute. But there are real constitutional disagreements in play between majority and minority.

The majority judgment makes a nod towards the importance of the prerogative in general but is dismissive of the significance of parliamentary accountability in particular. It is true that the Government’s accountability to Parliament is never a reason to expand prerogative power. But it may be highly relevant to understanding the propriety of a long-standing power, and hence a reason to be slow to conclude, in reading the 1972 legislation, that it has been set aside.

Lord Reed strongly implies, near the end of his judgment, that the majority has overlooked the constitutional importance of ministerial accountability to Parliament, that it has legalised a political issue, which is neither constitutionally appropriate nor wise. Likewise, Lord Carnwath notes that affirming parliamentary sovereignty is not a reason to overlook parliamentary accountability.

This legal claim should never have succeeded. The majority judgment makes some important mistakes in its handling of the relevant constitutional principles and the legal materials. It is unlikely to delay or obstruct Brexit if, as seems likely, the Commons and Lords support new legislation. The wider significance of the judgment – its potential to encourage other legal challenges to the use of the prerogative – remains to be seen.

But it is the Court’s unanimous dismissal of the devolution challenges that is more immediately significant for it avoids giving legal blessing to the arguments of the devolved administrations. Those arguments can still be made in the political arena but, as the majority rightly says, judges are neither the parents nor the guardians of constitutional convention.

Richard Ekins is Head of Policy Exchange's Judicial Power Project, and Tutorial Fellow in Law at St John's College, University of Oxford.