Theme: Government & Parliament | Content Type: Blog

Sovereignty, Legislative Supremacy and Devolution

Alison Young


Jamie Estevez

| 8 mins read

It is almost impossible to discuss the UK's constitutional arrangements without mentioning the sovereignty of Parliament. Any attempt by one Parliament to restrict the law-making powers of a future Parliament can be swiftly undone by the latter.

This said, ‘the sovereignty of Parliament, rather like the 'rule of law' is often played as the ultimate trump card in political debate. Nothing beats an argument which designates a constitutional change as modifying the sovereignty of Parliament. However, it can be difficult to discern what is really meant by this term.

This article outlines two possible interpretations of the sovereignty of parliament: parliamentary sovereignty and parliamentary legislative supremacy. A failure to appreciate this distinction obfuscates constitutional debates.

Sovereignty versus legislative supremacy

To describe the Westminster Parliament as sovereign is almost akin to ascribing omnipotence. On this account, the Westminster Parliament is the most important and powerful institution in the UK constitution. Its will overrides that of any other institution – be that the courts, an international organisation, or the devolved legislatures.

Parliamentary legislative supremacy focuses not on the institutions of the UK constitution, but on legal issues. If the Westminster Parliament is sovereign, it is because Acts of the Westminster Parliament are the highest form of law in the UK.

A failure to separate out these arguments seriously obfuscated debate over Brexit. The Westminster Parliament does now enjoy greater law-making power, being free to overturn retained EU law. Before Brexit, Westminster did not enjoy the same freedom to legislate contrary to directly effective provisions of EU law.

It appears, therefore, that sovereignty has been regained. Yet neither the UK’s membership of the EU nor Brexit altered the fact that Acts of the Westminster Parliament are hierarchically superior, overriding all other forms of UK law, even international Treaty obligations these Acts incorporated into UK law. The European Communities Act 1972 was the means through which directly effective provisions of EU law were able to disapply Acts of the Westminster Parliament during the UK’s membership of the EU. In a similar manner, since Brexit, Acts of the Westminster Parliament regulate the UK’s new relationship with the EU. This includes legislation to ensure the supremacy of those aspects of the Withdrawal Agreement that continue to have effect post Brexit, including protecting rights of EU citizens living in the UK and the Northern Ireland Protocol.

A.V. Dicey’s account of parliamentary sovereignty focuses on the hierarchical supremacy of Acts of Parliament. As noted above, in order to ensure that each Parliament’s Acts enjoy legislative supremacy, no single Parliament can legally limit the law-making power of its successor(s). In addition, no court, or body other than Parliament, can question a validly enacted Act of the Westminster Parliament.

Legal scholars may argue as to whether, whilst Parliament enjoys legislative supremacy, it can, or could during EU membership, legally restrict the law-making powers of future Parliaments. These legal niceties, however, miss the more fundamental issue. For Dicey, whilst Acts of the Westminster Parliament had legislative supremacy, the Westminster Parliament was not truly fully omnipotent. Politics, with its pressure from the electorate combined with the consciences of elected representatives, were a more legitimate restriction on Westminster’s omnipotence than any legal limits enforced by the courts.


The distinction between sovereignty and legislative supremacy matters. It requires us to ask why Parliament is sovereign. Do we have mechanisms of political control that provide legitimacy? Do they make Acts of the Westminster Parliament more or less legitimate than if the Westminster Parliament had limited sovereignty, with legally-entrenched constitutional limits over its powers?

In addition to these fundamental questions, an understanding of whether the Westminster Parliament is sovereign, or its Acts of Parliament enjoy legislative supremacy, affects the relationship between Westminster and the devolved legislatures and Governments. This is illustrated in a recent decision of the Supreme Court which concluded that Acts of the Scottish Parliament, designed to incorporate the United Nations Convention on the Rights of the Child and the European Charter of Local Self Government into Scottish law, were beyond the competences of Holyrood.

The decision of the Supreme Court is complex. It turns on a specific interpretation of whether these Acts of the Scottish Parliament modified the ability of the Westminster Parliament to continue to make law for Scotland. In line with its own earlier decision, the Supreme Court provided a broad account of when the ability of the Westminster Parliament to make law for Scotland would be modified. However, in doing so, it concluded that even provisions requiring Scottish courts to interpret legislation so as to comply with Treaty obligations or issuing a declaration of incompatibility when this was not possible, modified the UK Parliament’s ability to make law for Scotland. This broad account appears to fit more with an assessment of Westminster as a sovereign body, granting some of its ability to make law to the devolved legislatures.

Whilst this interpretation is possible – constitutional issues, including the devolved settlement, are reserved to Westminster – it does not match other aspects of the devolution settlement. Scotland has the power to modify Acts of the Westminster Parliament as they apply in Scotland unless that Act is restricted and cannot be so modified. This fits better with the idea that Westminster legislation is the highest form of law. Yet, if that is the case, how is Westminster’s ability to enact law for Scotland modified by requirements that legislation be interpreted in line with Treaty obligations, or declared incompatible, when this is not possible? Westminster can still enact legislation that contradicts these interpretations. It is an Act of the Westminster Parliament that determines the devolution settlement.

This may seem like splitting hairs. However, it raises deeper issues regarding the nature of devolution. The more we regard the Westminster Parliament as sovereign, and not just enjoying legislative supremacy, the more difficult it may be to facilitate a devolution settlement where Governments and legislatures work together for the greater good. To fail to do so may exacerbate tensions, requiring us to ask more fundamental questions as to whether political controls, such as the Sewel Convention (which states that the Westminster Parliament will not normally legislate on a matter that has been devolved to Scotland, Wales, or Northern Ireland without the consent of that devolved legislature), are sufficient, or whether there is a need to devise legal controls to maintain the Union.

  • Alison Young

    Alison Young

    Professor Alison Young is the Sir David Williams Professor of Public Law at the University of Cambridge, and a Fellow of Robinson College.

    Articles by Alison Young