Constitutional conventions

Constitutional conventions are non-legal constitutional rules. They are not merely habits, courtesies, or political preferences. They are standards of constitutional conduct regarded by relevant constitutional actors as binding, but not enforceable as law by the courts. Their importance in the United Kingdom follows directly from the features studied in earlier weeks: an uncodified constitution, parliamentary sovereignty, extensive prerogative powers, a politically accountable executive, and a constitution in which much of the working relationship between Crown, ministers, Parliament, devolved institutions and courts is regulated outside formal legal text.

The central difficulty is that conventions occupy a middle category. They are constitutional, because they structure the operation of legal powers. Yet they are not law, because a breach does not, without more, produce a legal remedy. The Prime Minister is appointed under a prerogative power, but convention determines who should be appointed. Royal Assent is a legal power, but convention determines that it is not refused to a Bill duly passed by Parliament. Ministers legally hold office under the Crown, but conventions of individual and collective responsibility determine the constitutional terms on which they answer to Parliament. The United Kingdom Parliament legally retains power to legislate for devolved matters, but the Sewel Convention states that it will not normally do so without devolved consent.

For Durham first-year purposes, conventions are a consolidation topic. They connect directly with Week 1 on sources, Week 2 on sovereignty, Week 5 on separation of powers, Week 6 on prerogative power, and Week 7 on devolution. They also prepare the ground for later public law work on justiciability, legitimate expectation, ministerial accountability, and constitutional remedies. A strong answer therefore avoids treating conventions as a self-contained curiosity. It asks how they mediate between law and politics.

Three propositions should anchor the topic. First, conventions can be recognised by courts as facts about the constitution. Secondly, courts will not enforce conventions as conventions. Thirdly, the existence of a convention may still matter legally where it forms the factual background to a legal question: for example, the scope of a statutory provision, the practical effect of prerogative action, or the relationship between legal power and constitutional principle.

The leading modern authority is R (Miller) v Secretary of State for Exiting the European Union [2017] UKSC 5. The Supreme Court recognised the Sewel Convention but refused to police it. That judgment is not a minor devolution point; it is the leading judicial restatement of the orthodox boundary between law and convention. The Court accepted that conventions have constitutional significance, but emphasised that their content and operation are determined within the political sphere. This is consistent with Madzimbamuto and Attorney General v Jonathan Cape, but Miller gives the modern formulation.

In examination terms, the best answers resist two simplistic errors. One is legal positivist overstatement: conventions are not law, therefore they do not matter. That is false. The other is constitutional moralism: conventions are important, therefore courts should enforce them. That is also false unless a legal source independently authorises judicial intervention. The skill is to maintain the distinction while showing how porous the boundary can become in practice.

The modern idea of constitutional convention is usually traced to A V Dicey. Dicey distinguished between the law of the constitution and the conventions of the constitution. The former consisted of rules enforced by the courts; the latter consisted of understandings, practices and maxims governing the conduct of constitutional actors. Dicey’s distinction was elegant, but his account of conventions was tied to his broader theory of parliamentary sovereignty and representative government. Conventions, for Dicey, were not ornamental. They ensured that legal powers were exercised in accordance with democratic expectations. The legal sovereign was Parliament; the political sovereign was the electorate. Conventions were part of the machinery connecting the two.

The nineteenth-century constitution made this distinction necessary. The Crown retained substantial legal powers: appointment and dismissal of ministers, dissolution of Parliament, assent to legislation, treaty-making, war and foreign affairs. If one looked only at legal form, the United Kingdom appeared monarchical. If one looked at constitutional practice, government was parliamentary and responsible. Conventions explained the discrepancy. The monarch acts on ministerial advice; ministers must command the confidence of the House of Commons; taxation and supply require Commons authority; ministers answer to Parliament; and the House of Lords traditionally exercises restraint in relation to financial measures and manifesto legislation. These rules were not created by statute, yet they were central to the working constitution.

The twentieth century altered the context without removing the need for conventions. The expansion of the administrative state increased the importance of ministerial accountability. The development of disciplined party government complicated the idea that Parliament controlled ministers through deliberation. The Parliament Acts 1911 and 1949 reduced the Lords’ legal veto, but conventions continued to regulate relations between the Houses. The Salisbury-Addison Convention, associated with the post-1945 settlement, stated that the House of Lords should not block at second or third reading government Bills implementing manifesto commitments. Its precise content has always been contested, and changes to the composition of the Lords have weakened its practical certainty. That illustrates a general feature: conventions are historically contingent and vulnerable to political change.

Devolution produced a new generation of conventions. The most important is the Sewel Convention, named after Lord Sewel during the passage of the Scotland Bill. It concerns the relationship between the UK Parliament and devolved legislatures. The convention was later given statutory recognition in the Scotland Act 1998, section 28(8), as inserted by the Scotland Act 2016. That statutory recognition raised a fundamental issue: does putting a convention into a statute transform it into law? Miller (No 1) answered no, at least in relation to Sewel. The wording recognised a political limitation but did not convert it into a legally enforceable restriction on Westminster’s legislative competence.

Recent political events have intensified interest in conventions. The Brexit process exposed strain in the relationship between government, Parliament and devolved institutions. The 2019 prorogation litigation concerned legal limits on a prerogative power, but the surrounding controversy was deeply convention-laden: responsible government requires Parliament to be able to scrutinise the executive, especially during a period of constitutional change. The courts in Miller (No 2) did not enforce a convention against prorogation; they identified a legal limit derived from constitutional principles. That distinction matters. Modern judicial review may translate some constitutional expectations into legal principles, but it does not follow that conventions themselves have become legally enforceable.

The historical lesson is therefore one of continuity and tension. Conventions arose to reconcile legal form with political reality. They still perform that function. But their stability depends on shared constitutional understandings among political actors. Where those understandings weaken, the courts face pressure to intervene. The orthodox answer remains that courts enforce law, not conventions; but courts increasingly confront cases in which the legal question cannot be understood without reference to the conventional constitution.

The first principle is the distinction between legal validity and constitutional propriety. A legal power may be validly exercised even if its exercise is unconstitutional in the conventional sense. If the monarch refused assent to a Bill passed by both Houses, that would be a grave breach of convention; the legal consequences would raise exceptional questions, but the convention itself is not an ordinary legal rule. If Parliament legislates for devolved matters without consent, the legislation is legally valid, notwithstanding breach or alleged breach of Sewel. Conversely, an act may comply with convention yet still be unlawful if it exceeds legal power. Conventions do not license illegality.

The second principle is Jennings’ classic test. Sir Ivor Jennings argued that three questions should be asked: what are the precedents; did the actors believe themselves bound by a rule; and is there a reason for the rule? This remains the most useful analytical framework. Precedent alone is insufficient, because repeated conduct may be no more than habit. A sense of obligation is essential, because conventions are normative practices. A reason is also essential, because conventions are justified by constitutional principle: responsible government, democratic legitimacy, avoidance of monarchical discretion, comity between institutions, or protection of devolved autonomy.

Jennings’ test is powerful but not mechanical. Some conventions arise from a single decisive precedent if the constitutional reason is strong and actors accept it as binding. Others persist despite uncertain precedents because their rationale is deeply embedded. Still others decline because political actors cease to accept them as obligatory. The Fixed-term Parliaments Act 2011, later repealed, temporarily altered the conventions governing dissolution. The repeal of that Act and enactment of the Dissolution and Calling of Parliament Act 2022 revived the prerogative-style position while attempting to exclude judicial review. The point is that conventions operate in a changing institutional environment; they are not frozen historical artefacts.

The third principle is that conventions regulate actors who possess legal powers. Conventions usually attach to the exercise of legally recognised powers rather than replacing those powers. The Crown has legal power to appoint a Prime Minister; convention directs appointment of the person best able to command confidence in the Commons. Ministers possess legal authority to advise the Crown; convention requires that advice be given by ministers who are politically responsible. Parliament has legal power to legislate; convention may regulate when that power should be exercised in relation to devolved matters. This explains why conventions are constitutionally significant despite lacking legal enforceability.

The case law on conventions is relatively sparse, because conventions are not ordinarily litigated. The leading cases are important less for detailed remedial doctrine than for the boundary they draw between law and politics.

Madzimbamuto v Lardner-Burke is the orthodox starting point. The Privy Council considered the legal consequences of Southern Rhodesia’s unilateral declaration of independence and the validity of emergency regulations made by the unlawful regime. The case contains the classic statement that a practice may be unconstitutional in a political sense without being legally impossible. Lord Reid’s reasoning is often used to show that conventions may impose powerful constitutional restraints while leaving legal power intact. The case supports a strict distinction between legal competence and constitutional propriety.

Attorney General v Jonathan Cape is the leading domestic case on judicial recognition of a convention. The Attorney General sought to restrain publication of Richard Crossman’s diaries, which disclosed Cabinet discussions. The court accepted the existence and importance of collective Cabinet responsibility, but the cause of action was not enforcement of that convention. The legal issue was whether publication should be restrained under equitable principles of confidentiality. The court refused an injunction because the material was no longer sufficiently confidential. The case is therefore a model of recognition without enforcement.

Manuel v Attorney General concerned the Canada Act 1982. Indigenous Canadian claimants challenged the UK legislation on the basis that constitutional conventions required Canadian provincial consent. The Court of Appeal rejected the challenge. The case demonstrates that even solemn constitutional understandings concerning imperial or post-imperial legislative practice do not limit Parliament’s legal authority unless embodied in law. It is a useful companion to Madzimbamuto and anticipates the reasoning later applied to Sewel.

R (Wheeler) v Office of the Prime Minister is not a pure convention case, but it is valuable for the limits of judicial control over political commitments. The claimant challenged the Government’s refusal to hold a referendum on the Lisbon Treaty after a manifesto commitment concerning the proposed Constitutional Treaty. The Divisional Court rejected the claim. Broad political promises do not ordinarily create enforceable legal expectations, particularly in matters of high policy and parliamentary process. The case helps students avoid conflating political accountability with legal enforceability.

R (Evans) v Attorney General concerned the Attorney General’s certificate vetoing disclosure of correspondence between the Prince of Wales and ministers. The litigation turned on statutory interpretation and constitutional principle, not enforcement of the heir-to-the-throne convention. Yet the case illustrates how conventions may form part of the constitutional background to legal disputes. The Prince’s correspondence had been justified partly by reference to his preparation for kingship; that convention became relevant context, even though the court did not enforce it as a freestanding rule.

Miller (No 1) is the central modern authority. The Supreme Court held that ministers could not use prerogative power to trigger Article 50 without statutory authorisation, because withdrawal from the EU would alter domestic law and affect statutory rights. On the devolution issue, however, the Court refused to treat the Sewel Convention as legally enforceable. The judgment is essential because it recognises the convention’s constitutional significance while denying judicial competence to police it.

Miller (No 2) should be used carefully. The case held that the prorogation of Parliament in 2019 was unlawful because it frustrated or prevented Parliament from carrying out its constitutional functions without reasonable justification. This was a legal principle controlling prerogative power, not enforcement of a convention that Parliament should sit. The case therefore shows how courts may articulate legal limits adjacent to conventions, but not because conventions have themselves become law.

See practice questions section below.

See further reading section below.