Among others, it raises questions about the nature of parliamentary sovereignty, the extent of the executive’s prerogative authority, the status within the domestic legal system of European Union law, the proper scope of the judicial role, and extent to which the contemporary territorial constitution, and the conventions relating to it, serve as a brake upon the powers of UK constitutional actors.

As is well known, the factual backdrop against which the Supreme Court was required to confront this panoply of issues relates to the question whether the process whereby the UK will withdraw from the EU under Art 50 of the Treaty on European Union can be triggered by the executive using prerogative power. The Supreme Court — like the Divisional Court before it — answered that question in the negative. However, the Supreme Court went further than the Divisional Court in two respects: by stipulating explicitly that an Act of Parliament will be needed before notification of the UK’s intention to leave the EU can be served under Art 50; and (in answer to a question that had not been before the Divisional Court) by holding that there is no legal requirement that the devolved institutions be consulted about or assent to the triggering of Art 50. On the devolution issue, the court was unanimous; on the question whether the prerogative could be used, only Lords Carnwath, Hughes and Reed dissented — the latter in a substantial and powerful judgment.

The prerogative and legislation

The availability of the prerogative in this context inevitably raises fundamental questions about its relationship with legislation — in the form primarily, but not exclusively, of the European Communities Act 1972 — and hence about the operation of the axiom of parliamentary supremacy upon the Crown’s inherent prerogative power. That the prerogative yields, where relevant, in the face of legislation is a constitutional principle as long-established as it is uncontroversial. Indeed, it flows from the fact — acknowledged over 300 years ago — that it is the Crown-in-Parliament, and only the Crown-in-Parliament, that can lay claim to sovereign legislative authority. However, the hard question raised by Miller was whether the triggering of Art 50 engaged that principle in the first place.

Central to the disagreement were questions about the 1972 Act and the domestic status accorded by it to EU law. The Justices in the majority acknowledged that the Act ‘is the source of EU law’ in the UK because ‘without that Act, EU law would have no domestic status’. Thus, said the majority, EU law owes its validity in the UK to the 1972 Act, rather than to any change in the ‘rule of recognition… by reference to which all other rules are validated’. Yet the majority also contended that ‘the institutions of the EU’ are themselves a source of UK law, such that EU law is ‘an independent and overriding source of domestic law’. What is sorely lacking from the majority’s analysis is any clear acknowledgment of — let alone attempt to resolve — the basic tension inherent within its analysis. Quite how EU law can be an independent source of UK law while being dependent for that status upon the 1972 Act is never revealed.

Yet it is upon that questionable premise that the remainder of the majority’s reasoning is built, for once EU law is conceived of as an independent source of domestic law, it stands to reason that the foreign relations prerogative cannot be used to initiate the removal of that source of (domestic) law or of the (domestic) rights accorded by it. Meanwhile, the majority seeks to buttress its position by relying upon the sheer scale of the constitutional implications of joining — and hence of leaving — the EU. According to the majority, ‘a major change to UK constitutional arrangements can[not] be achieved by a ministers alone’, meaning that withdrawal from the EU — being a ‘far-reaching change to the UK constitutional arrangements’ — cannot be accomplished by executive fiat.

Dissenting judgments

If the majority painted with broad strokes, Lord Reed — whose dissenting judgment is by far the most substantial of the three, and which commanded the support of the other dissentients — preferred a finer brush. The result is a judgment that some will doubtless consider formalistic. But the line between undue formalism and analytical rigour is a fine one — and Lord Reed’s dissent remains on the right side of it. The root of his disagreement with the majority lies in competing readings of the European Communities Act 1972 and the status thereby accorded to EU law within the UK system. Noting that the Act vouchsafes the domestic legal effect only of such rights as are provided for by the EU Treaties ‘from time to time’, Lord Reed concludes that the Act amounts to no more than a ‘scheme under which the effect given to EU law in domestic law reflects the UK’s international obligations under the Treaties, whatever they may be’. On this analysis, the Act is neutral as to whether any such obligations exist by dint of the UK being a member state: it ‘imposes no requirement, and manifests no intention, in respect of the UK’s membership of the EU’. The upshot is that while withdrawal under Art 50 ‘alters the application of the 1972 Act’, it ‘is not inconsistent with it’.

This reasoning gets Lord Reed to the conclusion that the use of the foreign relations prerogative for the purpose of triggering Art 50 would not be incompatible with the Act, because the conditionality hardwired into the scheme of the Act accommodates not only the possibility of the UK’s Treaty obligations varying over time, but also the possibility of those obligations ceasing to apply altogether. And, for Lord Reed, in contrast to the majority, the foreign relations prerogative is pertinent in the first place because the triggering of Art 50 remains a matter of such relations, EU law being distinct from and not an independent source of domestic law. Nor was Lord Reed moved by the argument of ‘scale’ that so impressed the majority, one aspect of which was their conviction that while the 1972 Act accommodates the removal of rights via EU legislation, it cannot accommodate ‘complete withdrawal’, the latter being ‘a change which is different not just in degree but in kind from the abrogation of particular rights, duties or rules derived from EU law’. Lord Reed — correctly in my view — concluded that there was ‘no basis in the language of the 1972 Act’ for distinguishing between ‘variations in the content of EU law arising from new EU legislation, and changes resulting from withdrawal by the UK from the European Union’.

The territorial constitution

One of the questions lying at the heart of Miller is the extent to which the application of fundamental constitutional principles can be transformed by the novelty or significance of the constitutional arrangements that are in play. It was the majority’s preparedness to treat EU membership as a constitutional game-changer that (at least in part) explains its willingness to view EU law’s relationship with domestic law in terms that precluded executive-initiated disturbance of the former. To some extent at least the other, territorial, dimension of the case can be viewed similarly. It is trite that, as a matter of realpolitik, the UK’s constitutional architecture has changed almost beyond recognition over the last two decades, thanks to the introduction and subsequent deepening of devolution. The question, however, was whether those radical changes in the political-constitutional order had legal implications that it was for the court to adjudicate upon. The court unanimously held that they did not.

In so concluding, the Justices who formed the majority on the prerogative question — and who considered the competing analysis to be inadequately ‘realistic’ — held that what they might have been expected to take as ‘constitutional reality’ did not impinge upon their analysis of the devolution issues. Key among those issues was the question whether the Sewel Convention — according to which the UK Parliament will normally legislate so as to, among other things, adjust devolved competence only if there is devolved consent — encroached upon the triggering of Art 50. The argument was that departure from the EU would rid devolved institutions of their obligation to abide by EU law, thereby expanding devolved competence and triggering the Convention. The court, however, declined to determine whether the Convention applied, holding that judges ‘are neither the parents nor the guardians of political conventions; they are merely observers’. It followed that while they can ‘recognise the operation of a political convention in the context of deciding a legal question’, they cannot ‘give legal rulings on its operation or scope, because those matters are determined within the political world’.

It is noteworthy that this conclusion was reached notwithstanding s 28(8) of the Scotland Act 1998 (as inserted, in the wake of the Scottish independence referendum, by s 2 of the Scotland Act 2016). That provision glosses s 28(7) — which stipulates that the conferral of legislative power on the Scottish Parliament ‘does not affect the power of the Parliament of the United Kingdom to make laws for Scotland’ — by providing that it is nevertheless ‘recognised’ that the UK Parliament ‘will not normally legislate with regard to devolved matters without the consent of the Scottish Parliament’. Whereas the majority was prepared to treat EU membership as having effected a form of constitutional reconfiguration that could not be reversed without an Act of Parliament, the analysis of the devolution question was markedly more conservative. The Sewel Convention remained precisely that — a convention, and not a matter for the court.

Had the court ruled otherwise, the effect would have been seismic — for both the politics of Brexit and legal analysis of the constitution. Yet the court’s conservatism on this point might, in the longer term, prove to be of no less profound significance. For one thing, it suggests that ‘legislative recognition’ of the Sewel Convention, like statutory acknowledgment of the ‘permanence’ of the Scottish devolved institutions (and soon, via the Wales Bill, their Welsh counterparts), is a political gesture devoid of legal value. That, in turn, raises questions about the extent to which the federalisation of the British constitution falls inevitably to be stymied by the immovable obstacle of the sovereignty of the Westminster Parliament.

Indeed, it is in the face of that principle that some of the tensions within the majority judgment in Miller are thrown into the sharpest relief, the sovereignty principle (so we are told) being so accommodating as to facilitate EU law’s status as an independent and overriding source of UK law, but so uncompromising as to render the Scotland Act’s provisions as to the Sewel Convention and the devolved institutions’ permanence a dead legal letter. It is easy to portray Miller as a ‘victory’ of sorts for legislative supremacy, particularly when the issues are viewed through the binary optic of Parliament versus the executive. But what Miller really does is to reveal the malleability of the doctrine of parliamentary sovereignty and the constitutional tensions that are internalised within it. The majority was correct when it said that: ‘One of the most fundamental functions of the constitution of any state is to identify the sources of its law.’ A further, closely related such fundamental function is to rationalise the interaction of different loci of authority. If Miller clarifies anything, it is that discharging those functions is made unusually challenging by the ramshackle nature of the British constitution — and that while recourse to the rhetoric of parliamentary sovereignty is inevitable in such cases, it can get one only so far. 

Contributor Professor Mark Elliott, Professor of Public Law, University of Cambridge; Fellow, St Catharine’s College, Cambridge