The following is a lightly edited version of a piece that was first published in the autumn/winter 2020 edition of Lauterpacht Centre News, the newsletter of the University of Cambridge’s Lauterpacht Centre for International Law. Lauterpacht Centre News can be downloaded via the Centre’s website.

Another autumn, another Brexit-related constitutional drama in the UK. A year ago, British politics was dominated by Parliament’s enactment of legislation designed to reduce the likelihood of a no-deal Brexit, the Government’s attempt to thwart such legislation by proroguing Parliament for five weeks, and the Supreme Court’s explosive judgment holding the prorogation to be unlawful. A year on, the introduction into Parliament of the Internal Market Bill — designed to ensure a functioning post-Brexit single market within the UK — has invited fresh controversy. As well as potentially destabilising the UK’s territorial constitution, the Bill, as Ministers have freely acknowledged, would equip the Government to make domestic regulations entirely at odds with aspects of the EU Withdrawal Agreement.

The politics entailed in disowning aspects of a treaty that the Government negotiated, agreed and then fought an election on less than 12 months ago are as noteworthy as they are bewildering. But this episode also casts light on, and is of a piece with, a long-standing phenomenon that has been discernible throughout the debate about the UK’s membership of and departure from the European Union. The argument advanced by those in favour of withdrawal from the EU was, at root, about ‘taking back control’: a mantra that reflected the perceived impossibility of reconciling EU membership with ‘sovereignty’.

A conundrum that was rarely explored by the main political protagonists was the relationship between the notions of State sovereignty and the domestic concept of parliamentary sovereignty. It is clear, however, that the latter notion loomed large in the debate, and that exception was taken by many Brexiters to the capacity of EU law to assume any degree of priority over UK legislation. Within this legal worldview lie the seeds of a narrative that regarded membership of the EU — and, by extension, the international legal order more generally — as a particular affront to a peculiarly British notion of sovereignty. It is the elision of the notions of State and parliamentary sovereignty that animates that view, the latter being regarded by some as a form of legal kryptonite that entitles the UK to shrug off the constraining effects of international law that lesser legal orders have no choice but to accept.

Precisely such thinking was on display in the purported legal justification for the Internal Market Bill that was offered by the UK Government. While acknowledging that ‘a state is obliged to discharge its treaty obligations in good faith’, the Attorney General went on to indicate that this proposition was somehow qualified by ‘the fundamental principle of parliamentary sovereignty’, with the result that the UK Parliament ‘can pass legislation which is in breach of the UK’s Treaty obligations’. Of course, this assertion is mundane as a matter of domestic law but quite irrelevant as a matter of international law, resting as it does on the apparent assumption that the concept of parliamentary sovereignty is somehow capable of transcending its necessarily domestic context so as to impinge upon the UK’s legal position as a State in international law.

Yet such thinking is commonplace in certain strands of UK political discourse. Prior to the 2016 referendum on EU membership the then Prime Minister David Cameron told Parliament that ‘asserting the sovereignty of this House is something that we did by introducing the European Union Act 2011’ (section 18 of which contained a so-called ‘sovereignty clause’) and that he was ‘keen to do even more to put it beyond doubt that this House of Commons is sovereign’. More recently, in relation to the Internal Market Bill, former Conservative Party leader Iain Duncan Smith argued that through section 38 of the EU (Withdrawal Agreement) Act 2020 — which states that ‘[i]t is recognised that the Parliament of the United Kingdom is sovereign’ — the UK Government had ‘reserved to themselves the right to make clarifications’ in respect of the Withdrawal Agreement and were therefore ‘quite within their rights’ to rely on section 38 as a basis for declining to implement parts of the Agreement. These arguments are united by the erroneous notion that the domestic concept of parliamentary sovereignty somehow bears upon the position of the UK as a State under international law, thereby supplying the UK with a ‘get out of jail free’ card to be played when it finds itself inconveniently bound by international obligations. This elision consistently bedevilled political discourse about the UK’s membership of the EU and has equally served to obfuscate debate about Brexit.

Criticism of the Internal Market Bill has so far resulted only in a limited concession from the Government, whereby a further vote in the House of Commons would be required to trigger the relevant ministerial powers. This, perhaps, is testament to the resonance of an exceptionalist argument that has long served the causes of Euroscepticism and Brexit and which plays well in political terms to certain domestic audiences. None of this, however, can change the fact — as international reaction has attested — that the argument disintegrates upon contact with legal reality. Brexiters’ kryptonite may be potent when used in the parochial context of domestic political debate, but its true nature is exposed, and its legal impotence revealed, by attempts to deploy it on a wider, international stage.