Brexit at 10: The courts and the law

Brexit was a seismic legal and constitutional change for the UK. It involved disentangling ourselves from decades’ worth of EU law, including all the legal principles and apparatus which went with it. That meant repealing the European Communities Act 1972 and removing the supremacy of EU law over national law, the concepts of direct applicability and direct effect of EU law, the role of the EU Court of Justice (CJEU) and the possibility that even primary legislation – an act of parliament – might be overturned if it was inconsistent with EU law.

The creation of ‘retained EU law’

The substance of all that accumulated EU law did not, though, disappear or change the moment we left the EU. That would have left a gaping void in vast areas of the UK’s legal framework, covering employment law, health and safety, the environment, transport, state aids, product safety and much else. Instead parliament enacted the European Union (Withdrawal) Act 2018 which created the concept of “retained EU law”, in effect taking a snapshot of all existing EU law and converting it into UK domestic law.

The 2018 Act also gave ministers powers to amend that body of law, by secondary legislation, so that it ‘worked’ once we had left the EU – for example, so that it ceased to define the UK as a “member state”, and conferred regulatory or enforcement functions on national rather than EU bodies. Many thousands of such instruments were made.

The Retained EU Law (Revocation and Reform) Act 2023 further reduced the status of retained EU law and (rather pointlessly) re-labelled it “assimilated law”. That act also gave ministers wide powers to revoke, amend or replace such law. The legislation originally contained a broad sunset clause that would have automatically repealed most of this assimilated law at the end of 2023. That again would have created major gaps and much legal uncertainty.

In the end greater sense prevailed and instead only a limited number of repeals were made – in many cases these were laws which had already expired anyway. Thus some 4,000 pieces of former EU law remain on the UK statute book with only minor, technical modifications.

Enter the courts

Brexit was legally as well as politically contentious. In the Miller 1 case, the High Court held ¹⁷ [2016] EWHC 2768 (Admin).  that ministers could not rely on the royal prerogative (the non-statutory powers of the Crown under which ministers normally enter into treaties and conduct international relations) to invoke the Article 50 process for leaving the EU, but that an act of parliament was required. The Daily Mail famously branded the three judges who decided the case “enemies of the people”.

The Supreme Court went on to uphold their decision ¹⁸ [2017] UKSC 5.  and parliament duly enacted the 2-clause EU (Notification of Withdrawal) Act 2017. In Miller 2 the legal issue was not strictly about Brexit itself, but the context was Boris Johnson’s decision to prorogue parliament for an extended period so that he could “get Brexit done”. The Supreme Court (11 justices, presided over by spider brooch-wearing Lady Hale) held that decision to be unlawful, because – with no reasonable justification – it prevented parliament from carrying out its constitutional functions. ¹⁹ [2019] UKSC 41.

However, the process of converting former EU law into national law worked, as intended, to provide continuity and stability in the UK’s legal system over the period of our departure and there has been relatively little litigation in the UK courts about the technicalities of that process. ²⁰ See e.g. Lipton v BA Cityflyer Ltd [2021] UKSC 454.  In practice, courts and tribunals are largely applying assimilated law in just the same way as they used to apply the predecessor EU law, with the same substantive results.

A more sovereign parliament?

For Brexiteers at least the idea was always that turning pre-existing EU law into domestic law was only a temporary measure: after Brexit, parliament – now truly sovereign once more – would be free to change or diverge from that law in any way it liked. In some areas that has happened, including: the ending of EU-derived free movement rights and the introduction of new immigration rules for EU nationals; banning the export of live animals; the imposition of VAT on school fees; and new UK statutes on subsidy control (what the EU calls state aid), financial services, procurement and some aspects of environmental law.

Still, in many instances the new legislation does not greatly diverge from EU law and sometimes adopts concepts familiar from it.

Where does that leave things now?

The legal and constitutional processes of Brexit, including the transition period that ran from 31 January to 31 December 2020, are complete. We have left. The UK is no longer party to the EU treaties. EU law has ceased to be automatically part of UK domestic law or have supremacy over it. The CJEU’s jurisdiction over the UK has ended (almost completely – though more of this below) and there is no longer a British judge or advocate general on that court.

So now the UK Supreme Court is indeed ‘supreme’ and has the last word in interpreting our laws. ²² Leaving aside the European Convention on Human Rights and the European Court of Human Rights: that is another court and another story.  Parliament has already legislated to change many areas of UK law in the period since our departure.

Has Brexit therefore fundamentally changed the role of the UK courts? Some things have obviously changed. UK judges are no longer applying EU law as such in the same way as when we were members. They are no longer making references to the CJEU for an authoritative ruling on the meaning of EU law. And there have been changes in personnel in both government and the higher courts (Lord Reed replaced Lady Hale as president of the Supreme Court in 2021) which may have affected at least some of the atmospherics in the relationship between government and judiciary.

But most judges would say they continue to do what they have always done – interpreting and giving effect to the law in accordance with the will of parliament. Before Brexit that meant applying EU law pursuant to the European Communities Act 1972: now it means applying such assimilated EU law as parliament has chosen to retain, and whatever new legislation it enacts.

Brexit has not of course removed all legal ties between the UK and the EU. In international law, the UK is bound by the Withdrawal Agreement and the Trade and Cooperation Agreement concluded as part of our exit. Those agreements provide for dispute resolution processes, and breach could ultimately lead to measures including compensation, suspension of obligations or retaliatory action. In the case of the Withdrawal Agreement, there is a residuary role for the CJEU if a dispute turns on the interpretation of an EU law concept.

The Northern Ireland question

Northern Ireland is a special case – most obviously because of its land border with the Republic of Ireland and the desire to avoid this becoming a hard border. Under the original Northern Ireland Protocol, and later the Windsor Framework, some aspects of EU law continue to apply to goods in Northern Ireland, and the CJEU retains a limited interpretative role. In addition, the Supreme Court has held that some aspects of Article 2 of the Windsor Framework – which requires the UK to ensure that no diminution of rights, safeguards or equality of opportunity results from the UK’s withdrawal from the EU – are capable of having direct effect in national law. ²⁴ Although not the paragraphs of Article 2 relied on in a challenge to the Northern Ireland Troubles (Legacy and Reconciliation) Act 2023: Dillon, [2026] UKSC 15.  Thus concepts of direct effect and perhaps, supremacy, may continue to have some relevance post Brexit. 

What lies ahead for the UK courts, politics and parliament?

The UK’s future legal relationship with the EU will depend on its political relationship. Any changes, up to and including membership of the single market (let alone rejoining), will require the agreement of both sides, the conclusion of new international agreements and no doubt domestic legislation. The 2026 King’s Speech promised a bill containing powers to implement new treaties with the EU on food and drink, emissions trading and electricity – as well as any further treaties the UK may conclude with the EU in future. The UK could also unilaterally opt for closer alignment with EU regulations and standards.

Either way, an important issue will be the role of parliament in approving and scrutinising any measures to implement new treaties or align with EU rules. If this is all to be done by regulations made by ministers – secondary legislation – it risks being seen as bringing in EU rules ‘by the back door’ without proper parliamentary scrutiny or democratic legitimacy.