Introduction
The United Kingdom’s withdrawal from the European Union, which was legally finalised on 31 January 2020, marked a fundamental change in the country's constitutional landscape. For nearly five decades, membership of the EU had a profound impact on UK law, most notably through the principle of the supremacy of EU law, which was given effect by the European Communities Act 1972 (ECA 1972). This principle required that UK domestic law be disapplied where it conflicted with EU law, a constitutional arrangement famously affirmed in the Factortame litigation (R v Secretary of State for Transport, ex p Factortame Ltd (No 2) [1991] 1 AC 603). Brexit was presented as an opportunity to “take back control” and restore the undiluted sovereignty of the UK Parliament. However, the process of disentanglement has been complex, and the influence of EU law has not been entirely erased. This essay will argue that while the formal supremacy of EU law has ended, its influence endures through the large body of ‘assimilated law’ that remains on the UK statute book. This influence is, however, being deliberately and systematically diminished by legislation, most significantly the Retained EU Law (Revocation and Reform) Act 2023, which has in turn created new constitutional tensions concerning the balance of power between Parliament and the executive.
The European Union (Withdrawal) Act 2018: A Bridge from the Past
The primary legislative instrument for managing the UK’s exit was the European Union (Withdrawal) Act 2018 (EUWA 2018). Its main objective was to ensure legal certainty and continuity by preventing a legislative ‘black hole’ from appearing the moment the UK left the EU (Barber et al., 2017). The Act achieved this through a "copy and paste" mechanism that preserved the existing body of EU law and incorporated it into the domestic legal system.
Section 1 of the EUWA 2018 repealed the ECA 1972, thereby formally ending the direct effect and supremacy of EU law in the UK. However, to maintain a functioning legal order, the Act created a new, distinct category of domestic law known as ‘retained EU law’ (REUL). Sections 3 and 4 of the Act took a snapshot of EU law as it stood on exit day and converted it into UK law. This included EU-derived domestic legislation (such as UK regulations implementing EU directives), direct EU legislation (like EU regulations which previously had direct effect), and other rights and principles recognised by the Court of Justice of the European Union (CJEU).
Crucially, section 6(3) of the EUWA 2018 initially stipulated that UK courts were bound to interpret REUL in accordance with the pre-exit case law of the CJEU. This meant that although the UK was no longer part of the EU’s legal order, the jurisprudence of the Luxembourg court would continue to hold significant sway over a large portion of UK law. This demonstrated a clear and direct continuing influence, designed to provide stability in the immediate post-Brexit period. While Parliament had formally re-asserted its sovereignty by passing the EUWA 2018, the substance and interpretation of a significant part of UK law remained tethered to its European origins.
The Gradual Shift: Modifying the Influence of EU Jurisprudence
The EUWA 2018 contained within it the seeds of future divergence. While lower courts remained bound by retained CJEU case law, the Act (as amended) granted the UK Supreme Court and the Court of Appeal the power to depart from it, using a similar test to that which the Supreme Court uses when departing from its own precedents. This created a mechanism for the UK’s highest courts to begin developing a distinct domestic interpretation of REUL.
The Court of Appeal considered the test for departing from retained CJEU case law in TuneIn Inc v Warner Music UK Ltd [2021] EWCA Civ 441. The court emphasised that legal certainty was a key consideration and that it would not depart from CJEU jurisprudence lightly. Lord Justice Arnold stated that departure would generally only be appropriate where the CJEU’s reasoning was “inconsistent with some fundamental principle of domestic law” or had been “shown to be unsatisfactory in some other way”. This cautious approach indicated that while the door to divergence was open, the judiciary was not planning to rush through it, recognising the value of continuity and predictability in the law. This judicial approach maintained the influence of EU legal thinking, albeit now subject to the final authority of the UK’s own senior courts.
The Retained EU Law (Revocation and Reform) Act 2023: An Accelerated Divergence
The most significant step in reducing the influence of EU law has been the enactment of the Retained EU Law (Revocation and Reform) Act 2023 (REUL Act 2023). This legislation signals a decisive policy shift from continuity to active divergence. Its primary effect is to fundamentally alter the status of the remaining body of REUL and provide the government with extensive powers to amend or remove it.
Firstly, the Act brought an end to the special status of EU law within the UK legal system. Section 4 abolished the principle of the supremacy of EU law, which had been preserved in a limited form by the EUWA 2018 for the interpretation of pre-Brexit statutes. Furthermore, section 5 abolished general principles of EU law, such as proportionality and legal certainty, as interpretive aids for domestic law from the end of 2023. These changes represent a definitive constitutional break, ensuring that all UK law is now subject to the ultimate authority of the UK Parliament. To symbolise this change, section 6 re-branded all remaining REUL as ‘assimilated law’.
Secondly, the REUL Act 2023 originally proposed a "sunset" clause that would have automatically revoked thousands of retained EU laws at the end of 2023 unless ministers acted to save them. Following widespread criticism regarding the potential for legal chaos, this was reversed. Instead, the Act revoked a specific list of around 600 pieces of legislation, while preserving the rest. However, and crucially for constitutional affairs, the Act grants ministers wide-ranging powers to amend, restate, or revoke assimilated law by statutory instrument. These are often referred to as ‘Henry VIII powers’ as they allow the executive to change primary legislation without the full parliamentary scrutiny that a new Act of Parliament would require. This has raised constitutional concerns about a shift in power from the legislature to the executive, potentially undermining the very parliamentary sovereignty that Brexit was intended to restore (Craig, 2023).
The Lingering and Indirect Influence
Despite the REUL Act 2023, the influence of EU law has not vanished. Substantively, thousands of laws originating from the EU remain in force as assimilated law, governing areas from environmental protection to workers’ rights. While these can now be changed, they form the baseline of UK regulation.
Furthermore, new forms of indirect influence have emerged through the international agreements that govern the UK’s post-Brexit relationship with the EU. The UK-EU Trade and Cooperation Agreement (TCA) contains ‘level playing field’ commitments, which oblige the UK not to lower its standards in areas like labour and environmental law in ways that could distort trade. While these do not give EU law direct effect, they act as a powerful political and economic constraint on the UK’s ability to diverge significantly from EU standards.
Most significantly, the Windsor Framework, which governs trade arrangements for Northern Ireland, means that a large body of EU law continues to apply directly in that part of the UK. This creates a differentiated constitutional arrangement within the United Kingdom, where one region remains dynamically aligned with the EU single market for goods. This demonstrates that EU law’s influence continues to have a direct and constitutionally significant impact on the territorial integrity of the UK’s internal market.
Conclusion
In conclusion, the enduring influence of EU law on the UK’s constitutional affairs is complex and multifaceted. The direct, formal supremacy of EU law, a defining feature of the UK constitution for decades, has been decisively ended by the EUWA 2018 and the REUL Act 2023. This has, in principle, restored the traditional doctrine of parliamentary sovereignty. However, the legacy of membership persists. The creation of retained EU law, now assimilated law, means that the substance of thousands of UK laws is still European in origin, and its interpretation will bear the imprint of CJEU jurisprudence for years to come.
The drive to remove this legacy through the REUL Act 2023 has itself created fresh constitutional questions, particularly regarding the extensive powers granted to the executive to alter the law. While direct influence has waned, indirect influence continues through the TCA and, more profoundly, through the special arrangements for Northern Ireland under the Windsor Framework. Therefore, while the UK has formally left the European Union’s legal order, EU law continues to cast a long shadow over its constitutional arrangements, shaping not only the content of its laws but also the ongoing debate about the balance of power between Parliament, the executive, and the judiciary.
References
Barber, N., Ekins, R. and Laws, J. (2017) Continuity, Change and Executive Power: A Report on the European Union (Withdrawal) Bill. [Online]. London: Policy Exchange. Available at: https://policyexchange.org.uk/wp-content/uploads/2017/09/Continuity-Change-and-Executive-Power.pdf (Accessed: 15 May 2024).
Craig, P. (2023) 'The Retained EU Law (Revocation and Reform) Act 2023: Substance, Process and Constitutional Ramifications', UK Constitutional Law Association Blog, 11 July. [Online]. Available at: https://ukconstitutionallaw.org/2023/07/11/paul-craig-the-retained-eu-law-revocation-and-reform-act-2023-substance-process-and-constitutional-ramifications/ (Accessed: 15 May 2024).
R v Secretary of State for Transport, ex p Factortame Ltd (No 2) [1991] 1 AC 603.
TuneIn Inc v Warner Music UK Ltd [2021] EWCA Civ 441.
Legislation
European Communities Act 1972.
European Union (Withdrawal) Act 2018.
Retained EU Law (Revocation and Reform) Act 2023.
