| Key terms Twin pillars: the term used to describe parliamentary sovereignty and the rule of law together, the two principles regarded as the joint foundation of the UK constitution. Parliamentary sovereignty: the principle, associated with the constitutional theorist A.V. Dicey, that Parliament can make or unmake any law it wishes, that no Parliament can bind a future Parliament, and that no other body, including the courts, can overturn an Act of Parliament. Rule of law: the principle, also associated with Dicey, that everyone, including the government, is subject to and equal before the law, and that no one can be punished except for a proven breach of the law decided by an ordinary court. Judicial review: in the UK, the process by which courts check that ministers and public bodies have acted lawfully and within their powers; unlike in the United States, it cannot be used to strike down an Act of Parliament itself. |
The historical development traced above produced a constitution with three defining features, best understood by contrasting each with the equivalent feature of the US Constitution.
- Uncodified: the UK constitution is not gathered into one document but is drawn from the five sources discussed below. The United States, by contrast, has been governed by a single written document, the US Constitution, since 1788.
- Unentrenched: because there is no codified higher law, constitutional change in the UK requires only the same simple parliamentary majority as any other statute, as shown by the repeal of the European Communities Act 1972 and by the House of Lords (Hereditary Peers) Act 2026. The US Constitution can only be amended through the much more demanding Article V process, requiring a two-thirds vote in both houses of Congress and ratification by three-quarters of the states.
- Unitary: legal sovereignty rests with the Westminster Parliament alone. Devolution to the Scottish Parliament, the Welsh Senedd and the Northern Ireland Assembly is a grant of power that Parliament retains the legal right to reclaim, even though the Sewel Convention means it will not normally legislate on devolved matters without consent. The United States is federal: sovereignty is constitutionally divided between the federal government and the states, and neither can unilaterally strip the other of its powers.
These three features rest on the twin pillars of parliamentary sovereignty and the rule of law, which work together in practice rather than separately.
- Parliamentary sovereignty means the courts cannot strike down an Act of Parliament, however unreasonable it might seem, because Parliament is the supreme law-making body. This is the sharpest contrast with the United States, where the doctrine of judicial review, established in Marbury v Madison (1803), allows the Supreme Court to declare a law unconstitutional and therefore void if it conflicts with the codified constitution.
- The rule of law constrains not Parliament itself but the executive, by requiring ministers to act within the powers Parliament has actually given them. This was demonstrated in Miller (No.1) (2017), when the Supreme Court ruled that the government could not trigger Article 50 to begin the Brexit process using prerogative power alone, and instead required an Act of Parliament.
- Because UK judges cannot overturn primary legislation, the rule of law in the UK operates mainly as a check on how governments exercise the powers Parliament has granted them, whereas in the United States the judiciary can constrain the legislature itself, not just the executive, giving the courts a formally stronger constitutional role.
| Check your understanding 1. Define parliamentary sovereignty in your own words, using one example from the historical documents studied above. You could: Include the three-part Dicey definition (Parliament can make or unmake any law, no Parliament can bind a successor, no other body can override an Act) and anchor it to a specific example, such as the Bill of Rights 1689. 2. Using the example of Miller (No.1) (2017), explain how the rule of law can constrain the government even without a codified constitution. You could: Set out what the government tried to do, what the Supreme Court ruled, and then link the outcome explicitly back to the rule of law principle that ministers must act within the powers Parliament has actually given them. 3. To what extent does the difference between UK and US approaches to entrenchment explain why judicial review works differently in each country? You could: Contrast the UK courts’ inability to strike down statute with the US Supreme Court’s power under Marbury v Madison, then consider whether entrenchment is the main explanation or whether other factors (such as codification itself) also matter, before giving a judgement. |
A fuller comparison of the separation of powers and checks and balances under the US Constitution is developed elsewhere on histpology.co.uk.
next – the five sources of the UK Constitution