So far in this series of posts and accompanying videos, I have introduced an overarching question about the nature of the UK constitution — and, in particular, about whether it is really as flexible as is commonly supposed — and have considered that question in relation to judicial review, the protection of human rights and the UK’s territorial constitutional arrangements. In this fifth, and penultimate, part of the series, I consider what we can learn about the UK’s constitution from its membership (and, latterly, departure from) the European Union.

When discussing the territorial constitution, I concluded by suggesting that we can detect a paradox in that context: namely, that while parliamentary sovereignty is in one sense the ultimate guarantor of the constitution’s flexibility, the rigidity of the sovereignty principle itself may stand in the way of changes — such as a more genuine sharing or pooling of authority among the UK’s constituent territories — that might help to make the overall constitutional structure a less brittle one that can better accommodate growing demands for a less top-down approach to governance. When we turn to the UK’s membership of and exit from the EU, it is arguable that a different paradox emerges: the UK constitution’s interaction with the legal structures of the EU has served to reveal that the notion of parliamentary sovereignty is more malleable than had been previously assumed, yet the case law that reveals that malleability simultaneously identifies other featuresof the constitution that have a relatively durable quality (in that they may be resistant to exercises of parliamentary sovereignty).

At the core of this issue is the relationship between parliamentary sovereignty and the primacy of EU Law. In attempting to characterise that relationship, it is helpful — at least for the purpose of setting up the discussion — to consider two relatively extreme forms that it might take. On one view, we might see parliamentary sovereignty as having been an absolute obstacle to the acknowledgment of EU primacy. Such an approach would clearly emphasise the durability of this aspect of the domestic constitution — but would have carried the inconvenient consequence that full compliance with the requirements of EU membership had always been legally impossible for the UK. At the other extreme, we might regard EU primacy as something that overwhelmed the domestic sovereignty doctrine — in effect extinguishing it, at least for the duration of the UK’s membership of the EU. This is essentially the position adopted by Sir William Wade, who argued that there had been a constitutional ‘revolution’.

Perhaps unsurprisingly, domestic courts adopted an intermediate position — one that ascribes a form of primacy to EU law, but on terms that are compatible with (a particular view of) the domestic sovereignty doctrine. To begin with, the UK courts’ approach was a highly pragmatic one that paid little attention to questions of constitutional theory. Later, however, in cases such Thoburn v Sunderland City Council [2002] EWHC 195 (Admin), [2003] QB 151 and R (HS2 Action Alliance Ltd) v Secretary of State for Transport [2014] UKSC 3, [2014] 1 WLR 324, the courts showed greater willingness to engage with deeper constitutional questions in this area.  The HS2 case is particularly noteworthy, since it carries the implication that certain constitutional norms may enjoy a particular degree of fundamentality that placed them beyond the possibility of interference by EU law and makes them at least resistant, if not impervious, to interference by primary legislation enacted by the UK Parliament. This raises significant questions as to the potential extent of the fundamentality of any given constitutional norm — and, in particular, about whether any norms may exist that are not merely resistant but impervious to legislative disturbance.