Introduction
The United Kingdom’s withdrawal from the European Union created a significant legal challenge: how to manage the vast and complex body of EU law that had accumulated over decades and was woven into the fabric of domestic law. The initial solution, established by the European Union (Withdrawal) Act 2018 (EUWA 2018), was to create a new category of law known as 'retained EU law' (REUL). This was designed to ensure continuity and prevent a legal ‘black hole’ on exit day. However, this was always seen by some as a temporary measure. The subsequent enactment of the Retained EU Law (Revocation and Reform) Act 2023 (REUL Act 2023) marked a decisive shift, replacing the concept of REUL with 'assimilated law' and fundamentally altering its status and interpretation.
This essay will argue that the move from retained EU law to assimilated law has not improved legal certainty in the UK. While the stated aim of the 2023 Act was to restore parliamentary sovereignty and provide ultimate constitutional clarity, its methods have introduced significant new areas of unpredictability. This essay will first outline the relative certainty provided by the initial REUL framework under the EUWA 2018. It will then analyse how the REUL Act 2023, by abolishing the principle of supremacy, removing general principles of EU law, and granting new powers to courts and ministers, has created a period of legal flux. It will be concluded that, in the short to medium term at least, the transition to assimilated law has undermined, rather than enhanced, the predictability and stability of the law.
The Cautious Certainty of Retained EU Law
The primary objective of the EUWA 2018 was to provide a functioning statute book on the day after the UK left the EU (The National Archives, 2018). It achieved this by taking a ‘snapshot’ of the EU law that was applicable in the UK on 31 December 2020 and preserving it in domestic law as ‘retained EU law’. This body of law included EU-derived domestic legislation, direct EU legislation (such as regulations), and the rights and principles established in EU case law.
For the purposes of legal certainty, the framework established by the EUWA 2018 had several key features that promoted stability. Firstly, and most significantly, section 5(2) of the Act preserved the principle of the supremacy of EU law for the pre-exit period. This meant that any domestic law passed before the end of the transition period that was inconsistent with a piece of REUL would be disapplied, just as it had been when the UK was an EU member. This provided a clear and familiar hierarchy for lawyers, judges, and businesses navigating existing legal obligations (Craig, 2021). While constitutionally novel, this rule offered predictability by ensuring that the status of thousands of regulations in areas from environmental protection to workers’ rights remained unchanged.
Secondly, section 6(3) of the EUWA 2018 preserved the ‘general principles of EU law’, such as proportionality, legal certainty, and legitimate expectations. These principles were essential interpretive tools that UK courts used to understand the meaning and scope of REUL. Their retention meant that the existing body of case law from the Court of Justice of the European Union (CJEU) remained relevant in interpreting these laws, providing a consistent methodological approach (Elliott, 2022).
Finally, the EUWA 2018 established a clear but cautious approach to judicial precedent. Section 6(4) stated that domestic courts were not bound by any post-exit decisions of the CJEU, but that they could have regard to them. For pre-exit CJEU case law, UK courts were to treat it as binding, with only the Supreme Court (and later the Court of Appeal) having the power to depart from it, using the same test they would for departing from their own previous decisions. This created a stable foundation of case law, allowing for gradual, considered evolution rather than abrupt change. The Court of Appeal in TuneIn Inc v Warner Music UK Ltd [2021] EWCA Civ 441 demonstrated this caution, declining to depart from retained case law and emphasising the importance of legal certainty. This system, while constitutionally awkward for a non-EU member state, prioritised continuity and predictability in the immediate post-Brexit period.
The REUL Act 2023: A Deliberate Shift Towards Uncertainty
The REUL Act 2023 was driven by a political desire to complete the process of Brexit by severing the remaining legal ties to the EU. The government’s aim was to ‘end the special status of retained EU law’ and reassert the sovereignty of the UK Parliament as the ultimate source of domestic law (HM Government, 2022). To achieve this, the Act fundamentally dismantled the framework created by the EUWA 2018. It renamed REUL as ‘assimilated law’ to signify its new status as a purely domestic form of law, but the changes went far beyond nomenclature.
The Act initially contained a controversial ‘sunset clause’ which would have automatically revoked the majority of REUL at the end of 2023 unless it had been specifically saved. This proposal was met with widespread criticism from legal and business communities, who warned it would lead to chaos and profound legal uncertainty, as thousands of laws could disappear overnight (House of Lords Constitution Committee, 2022). In a significant concession, the government amended the Bill, replacing the sunset clause with a schedule listing the specific pieces of legislation to be revoked at the end of 2023. While this averted the worst-case scenario, the philosophy of radical change underpinning the Act remained, and its other provisions have introduced new and challenging forms of uncertainty.
The New Uncertainties of Assimilated Law
The transition to assimilated law has damaged legal certainty in three principal ways: by creating ambiguity over the law’s hierarchy and interpretation, by encouraging the re-litigation of established legal principles, and by handing extensive and uncertain powers to government ministers.
The Loss of Supremacy and General Principles
The most significant change made by the REUL Act 2023 is the abolition of the principle of the supremacy of EU law. Section 4 of the Act explicitly states that from the end of 2023, the principle is no longer part of UK law. This means that assimilated law no longer has priority over pre-2021 Acts of Parliament. While this achieves the constitutional goal of restoring parliamentary sovereignty, it creates practical legal uncertainty. It is now unclear how conflicts between a piece of assimilated law and an older piece of domestic legislation will be resolved. While normal principles of statutory interpretation (such as specific provisions overriding general ones, or newer laws overriding older ones) will apply, the application of these rules to the complex and often overlapping body of assimilated and domestic law is untested and likely to generate litigation (Lakin and Reynolds, 2023).
Simultaneously, section 4 also abolished the general principles of EU law. These principles were not just abstract concepts; they were a vital part of the judicial toolkit for interpreting EU-derived law. Without them, courts are left in a "significant interpretative vacuum" (House of Lords Constitution Committee, 2022, para 55). For example, a regulation concerning consumer rights that was previously interpreted in light of the principle of proportionality must now be interpreted without it. It is unclear what will fill this gap. Courts may eventually develop new common law principles, but this process will be slow and piecemeal, creating a prolonged period of uncertainty for anyone trying to understand their legal rights and obligations under assimilated law.
The Expanded Power to Depart from Precedent
The REUL Act 2023 also seeks to accelerate the divergence of UK law from EU precedent. Section 6 of the Act lowers the threshold for courts to depart from retained EU case law. The power, once limited to the Supreme Court and Court of Appeal, is now extended to lower courts, including the High Court, in certain circumstances. More importantly, the Act introduces a new statutory test for making a departure. A court must consider factors including whether the retained case law is "no longer appropriate" in the domestic context and "the extent to which the retained domestic case law is determined or influenced by the principle of the supremacy of EU law" (s.6(5)).
This actively invites lawyers to challenge previously settled points of law. The vague nature of the "no longer appropriate" test gives judges considerable discretion, making legal outcomes harder to predict (UK in a Changing Europe, 2023). Businesses and individuals can no longer rely on a stable body of case law. Instead, they face the prospect of costly and uncertain litigation to re-establish legal principles that were once taken for granted. This measure, designed to hasten legal divergence, directly trades legal certainty for judicial flexibility.
Ministerial Powers and "Restated" Law
A final source of uncertainty stems from the very wide powers the REUL Act 2023 grants to government ministers. Sections 12 and 14 give ministers the power to revoke and replace assimilated law using secondary legislation, which typically receives less parliamentary scrutiny than primary legislation. This creates an environment where significant areas of law, from employment regulations to financial services rules, can be altered by ministers at speed, making it difficult for businesses and citizens to keep track of the current law.
Furthermore, section 15 grants ministers a power to 'restate' assimilated law. The intention is to make the law clearer and more accessible. However, the Act states that a restatement can change the law in order to "resolve ambiguities, doubts or other uncertainties" (s.15(4)(b)). This creates a serious problem for legal certainty. A minister may make what they consider a minor clarification, but which could be interpreted by others as a substantive change in the law. This power allows the executive to alter the law under the guise of tidying it up, creating a new, uncertain version of law that has not been subject to full parliamentary debate (Elliott, 2023). The result is that even when a law is not revoked, its meaning can be altered, adding another layer of unpredictability to the post-Brexit legal landscape.
Conclusion
The move from retained EU law to assimilated law, brought about by the REUL Act 2023, was presented as a necessary step to restore national sovereignty and provide long-term constitutional clarity. However, the legal and practical consequences of this transition have demonstrably reduced legal certainty. The initial framework under the EUWA 2018, though constitutionally imperfect, offered a high degree of stability by preserving the established hierarchy of laws, interpretive principles, and case law. It provided a predictable environment for businesses, lawyers, and citizens in the immediate aftermath of Brexit.
By contrast, the REUL Act 2023 has dismantled this stable foundation. The abolition of supremacy and the general principles has created an interpretive and hierarchical vacuum, inviting legal challenges to resolve the resulting conflicts and ambiguities. The new, wider powers for courts to depart from precedent will likely lead to an increase in litigation over previously settled matters, making legal outcomes less predictable. Finally, the broad powers granted to ministers to amend, revoke, and 'restate' law via secondary legislation create a volatile legal environment where the rules can change rapidly and with limited scrutiny. While the long-term ambition may be to create a more coherent and purely domestic legal system, the path chosen to get there has been one of deliberate disruption. In the current landscape, the move to assimilated law has therefore failed to improve legal certainty; it has actively undermined it.
References
Craig, P. (2021) The European Union (Withdrawal) Act 2018: The Key Constitutional Issues. In: Dougan, M., Gormley, L. and Spaventa, E. (eds.) The UK's Withdrawal from the European Union: A Legal Analysis. Hart Publishing.
Elliott, M. (2022) The Retained EU Law (Revocation and Reform) Bill 2022: Sunsetting, Supremacy and Scrutiny. Public Law for Everyone. [Blog] Available at: [https://publiclawforeveryone.com/2022/10/12/the-retained-eu-law-revocation-and-reform-bill-2022-sunsetting-supremacy-and-scrutiny/](https://publiclawforeveryone.com/2022/10/12/the-retained-eu-law-revocation-and-reform-bill-2022-sunsetting-supremacy-and-scrutiny/)
Elliott, M. (2023) The Retained EU Law Bill: A Threat to the Rule of Law? UK in a Changing Europe. [Online] Available at: [https://ukandeu.ac.uk/the-retained-eu-law-bill-a-threat-to-the-rule-of-law/](https://ukandeu.ac.uk/the-retained-eu-law-bill-a-threat-to-the-rule-of-law/)
European Union (Withdrawal) Act 2018, c. 16.
HM Government (2022) Retained EU Law (Revocation and Reform) Bill: Explanatory Notes. [Online] Available at: [https://publications.parliament.uk/pa/bills/cbill/58-03/0144/220144en.pdf](https://publications.parliament.uk/pa/bills/cbill/58-03/0144/220144en.pdf)
House of Lords Constitution Committee (2022) Retained EU Law (Revocation and Reform) Bill, 11th Report of Session 2022–23, HL Paper 107.
Lakin, S. and Reynolds, S. (2023) The Retained EU Law (Revocation and Reform) Bill in the House of Lords. UK Constitutional Law Association Blog. [Blog] 3 May. Available at: [https://ukconstitutionallaw.org/2023/05/03/stuart-lakin-and-stephen-reynolds-the-retained-eu-law-revocation-and-reform-bill-in-the-house-of-lords-the-government-survives-a-rebellion-but-must-move-quickly-to-address-the-bills-many-flaws/](https://ukconstitutionallaw.org/2023/05/03/stuart-lakin-and-stephen-reynolds-the-retained-eu-law-revocation-and-reform-bill-in-the-house-of-lords-the-government-survives-a-rebellion-but-must-move-quickly-to-address-the-bills-many-flaws/)
Retained EU Law (Revocation and Reform) Act 2023, c. 28.
The National Archives (2018) Explanatory Notes to the European Union (Withdrawal) Act 2018. [Online] Available at: [https://www.legislation.gov.uk/ukpga/2018/16/notes/division/1](https://www.legislation.gov.uk/ukpga/2018/16/notes/division/1)
TuneIn Inc v Warner Music UK Ltd [2021] EWCA Civ 441.
UK in a Changing Europe (2023) What is the Retained EU Law Bill?. [Online] Available at: [https://ukandeu.ac.uk/what-is-the-retained-eu-law-bill/](https://ukandeu.ac.uk/what-is-the-retained-eu-law-bill/)
