Like the decision of the High Court before it, the majority judgment begins by reciting a set of orthodox constitutional principles. ‘Parliamentary sovereignty is a fundamental principle of the UK constitution’, ([43]) which constrains courts and the executive. While the executive has prerogative power to enter into and withdraw from treaties, this does not, and cannot, change domestic law. So far, nothing new — though this is an important rejoinder to those who dispute the sovereignty of Parliament, or suggest that it can be repudiated by judges acting alone.

The majority’s treatment of the European Communities Act 1972 (ECA) seems far more novel. While the Court properly refrained from commenting on the wisdom or otherwise of Brexit, the majority’s reasoning emphasises the fundamental integration of EU and UK law and legal institutions — and hence the enormity of the UK exiting the union.

The majority describe the ECA as ‘the “conduit pipe” by which EU law is introduced into UK domestic law.’ More importantly, the ECA made EU institutions a source of EU law — and so it ‘provided a new constitutional process for making law in the United Kingdom’. As a result of this Act together with the EU Treaty, ‘a dynamic, international source of law was grafted onto, and above, the well-established existing sources of domestic law: Parliament and the courts’. Thus the act of giving notice under Article 50 will not merely change the law, but also the constitutional arrangements that determine how law is made.

So read, the judgment represents a striking extension of traditional constitutional ideas to the institutional arrangements of the EU. Though the majority clearly endorse long-standing constitutional principles, they also emphasise the fluidity of the UK constitutional structure. The decision therefore reminds me — an Australian public lawyer — of salient differences between the constitutional arrangements of the UK and those of Australia, where this kind of fluidity does not exist.

Dr Lisa Burton Crawford is a Lecturer at the Faculty of Law, Monash University. This post draws upon research conducted with Associate Professor Patrick Emerton of Monash University.

Click here for further analysis of the Supreme Court’s Miller judgment.