Short answer
Parliamentary sovereignty remains the constitutional starting point: Westminster can enact or repeal primary legislation, and UK courts do not ordinarily invalidate an Act of Parliament. The rule of law nevertheless requires government to act within legal authority, preserves access to independent courts and supports strict interpretation of provisions that interfere with fundamental rights or judicial review.
Brexit did not simply delete EU-derived rules. The European Union (Withdrawal) Act 2018 preserved major categories of law, now called assimilated law. Since the end of 2023, however, the former general priority of EU law, retained general principles and most rights previously saved through section 4 of that Act have ceased to be part of domestic law. A separate regime under section 7A still gives domestic effect and priority to rights and obligations arising under the Withdrawal Agreement, including relevant Windsor Framework rules.
The Human Rights Act 1998 and common law operate alongside those statutes. Courts interpret legislation compatibly with Convention rights where possible, review incompatible public-authority action and protect constitutional rights through the principle of legality. But an incompatibility declaration does not nullify primary legislation. The correct answer therefore depends on the legal source, date, territory, decision-maker and remedy—not a single abstract hierarchy.
Parliamentary sovereignty is the baseline, not a licence for executive government
The UK has no single codified constitutional instrument with entrenched superior status. Its constitution combines Acts of Parliament, common law, prerogative powers, conventions and institutional practice. In , the Supreme Court described parliamentary sovereignty as:
“a fundamental principle of the UK constitution”
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That principle means the Crown in Parliament makes primary legislation. Courts identify and apply it; they do not develop common law inconsistently with it. It also means ministers cannot change domestic law merely by prerogative action. Miller held that ministers could not trigger Article 50 without legislation because withdrawal would remove statutory rights created through the European Communities Act 1972.
The distinction is crucial. A court may quash an executive decision, subordinate instrument or devolved enactment because its maker exceeded legal authority. That enforces Parliament's legislation; it does not establish judicial supremacy over Parliament. Interpretation and Human Rights Act declarations likewise do not generally erase a Westminster Act.
Constitutional statutes may resist implied repeal, but Parliament can legislate expressly. Judicial remarks about hypothetical limits in an extreme case are not a settled power to strike down primary legislation.
The rule of law controls power through legality and courts
preserves:
“the existing constitutional principle of the rule of law”
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The rule of law informs legality, legal certainty, fair procedure, access to courts and judicial review. Government needs lawful authority for coercive action. Delegated powers must be exercised for their statutory purpose, rationally and fairly.
In , the Supreme Court invalidated employment-tribunal fees that effectively prevented access to justice. The judgment treated access to courts as inherent in the rule of law: statutory rights are of little value if people cannot enforce them.
In , the Court held prorogation unlawful because it frustrated Parliament's constitutional functions without reasonable justification. Political accountability did not replace judicial review. In , the majority strongly presumed against excluding review of legal error.
These cases primarily constrain executive and delegated power. Even forceful rule-of-law reasoning must be separated from the different question whether clear primary legislation controls the result.
Retained EU law became assimilated law
The avoided a regulatory cliff edge when EU law ceased to apply generally. It preserved EU-derived domestic legislation, converted applicable direct EU legislation and preserved relevant pre-completion case-law effects. Later UK and devolved legislation could amend that body of law.
The then changed the framework. At the end of 2023 it:
- revoked the instruments specifically listed in Schedule 1, rather than every EU-derived rule;
- repealed section 4 of the 2018 Act and the rights, powers and liabilities preserved solely through it, subject to savings and restatements;
- ended the general domestic principle of EU-law supremacy and retained general principles for post-2023 matters; and
- renamed remaining retained EU law, direct legislation and case law as assimilated law, assimilated direct legislation and assimilated case law.
The amended priority rule says assimilated direct legislation is read compatibly with ordinary domestic enactments where possible and is subject to them where inconsistent, unless a specific exception applies. Courts can make tailored incompatibility orders. Revised criteria and a reference procedure govern departure from assimilated case law.
The label alone answers little. Identify whether the rule is a UK implementing instrument, assimilated direct legislation, assimilated case law, a later replacement or a revoked provision. The event date matters because the former approach is preserved for pre-2024 acts or events.
Withdrawal Agreement law is a separate and continuing route
The removal of general supremacy did not remove the domestic effect of the EU–UK Withdrawal Agreement. gives qualifying rights, obligations, remedies and procedures arising under that Agreement legal effect in the UK and requires domestic law to be read subject to them.
In , the Supreme Court held that Parliament had authorised section 7A to modify the operation of inconsistent earlier legislation in relation to the Northern Ireland Protocol. That does not contradict sovereignty: the domestic priority comes from Parliament's own statute. Parliament remains legally capable of changing domestic law, but courts should not infer such a change from vague or unrelated wording.
This route matters especially for citizens' rights and the Windsor Framework. Northern Ireland has a distinct relationship with specified EU rules, so check the Framework, territorial extent and later UK legislation rather than assuming the Great Britain position.
The Human Rights Act changes interpretation and remedies, not sovereignty
The incorporates a structured domestic rights regime. Its principal mechanisms are:
- section 2: courts take relevant Strasbourg authorities into account but are not formally bound;
- section 3: legislation must, so far as possible, be read compatibly;
- section 4: specified higher courts may declare primary legislation incompatible;
- section 6: incompatible public-authority action is unlawful unless primary legislation required it; and
- sections 7 and 8: a victim may seek an appropriate remedy.
Section 4 expressly provides that a declaration:
“does not affect the validity, continuing operation or enforcement”
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of the provision. The incompatible Act therefore continues unless another remedy applies. Parliament may amend it, but the declaration does not compel repeal. Incompatible delegated legislation can often be quashed where the parent Act does not require the incompatibility.
Devolution statutes limit legislative and executive competence, including through Convention rights. An enactment outside competence can be invalid even though a Westminster Act normally cannot. Westminster remains sovereign; the Sewel convention is politically important but not judicially enforceable.
Common-law constitutional rights fill gaps and guide interpretation
The common law protects access to courts, liberty, open justice, procedural fairness, legal professional privilege and freedom of expression, among other interests. Under the principle of legality, Parliament is presumed not to override a fundamental right through general or ambiguous language. Clear words are required.
The principle normally determines statutory meaning or the limits of public power; it does not permit disregard of clear primary legislation. Common law may resolve a dispute first. The Act may then add proportionality, positive obligations and defined remedies.
A practical order of analysis
For a post-Brexit constitutional dispute, work through the following sequence:
- identify the challenged act and its maker: Westminster, a devolved legislature, minister, regulator or court;
- identify the territory, material date and exact statutory version;
- classify every EU-derived rule as domestic implementing legislation, assimilated direct legislation, assimilated case law, later replacement or Withdrawal Agreement law;
- test vires, ordinary statutory interpretation, common-law rights and the principle of legality;
- apply Human Rights Act sections 2, 3 and 6, then identify the available remedy under sections 4, 7 and 8; and
- state separately what can be quashed, disapplied, declared incompatible or left for legislative correction.
Conclusion
The post-Brexit constitution is plural but not formless. Parliamentary sovereignty identifies the ultimate domestic lawmaker. The rule of law ensures that executive and delegated powers remain legally bounded and reviewable. Assimilated law continues because Parliament preserved it, but no longer carries the former general EU-law supremacy and principles. Withdrawal Agreement law retains a distinct statutory priority. The Human Rights Act and common-law rights shape interpretation, review and remedies without ordinarily invalidating a clear Westminster Act. Reliable advice therefore begins with classification and remedy, not slogans about which source is “supreme”.
Sources
This article provides general legal information, not advice for a specific matter. Laws, facts and procedural deadlines may change the result. Follow the linked primary sources and obtain local advice before acting.