Separation of powers

Separation of powers is not a single rule in the United Kingdom constitution. It is a family of principles governing the allocation, restraint and accountability of public power. In a codified constitution such as that of the United States, it is possible to begin with textual allocations: legislative power in Congress, executive power in the President, judicial power in the courts. In the United Kingdom, the starting point is different. The constitution is historical, partly statutory, partly common law, and partly conventional. The question is therefore not whether powers are separated in a pure Montesquieuian sense. They are not. The question is which separations matter, why they matter, and by what legal techniques they are maintained.

The orthodox British model contains three central institutions. Parliament enacts primary legislation and authorises taxation and expenditure. The executive governs: it proposes policy, administers statutory schemes, conducts foreign affairs, deploys prerogative powers and commands the civil service. The courts decide disputes according to law and review the legality of executive action. Yet these categories immediately overlap. Ministers sit in Parliament and depend upon the confidence of the House of Commons. The executive normally controls the legislative timetable. Judges develop the common law and interpret statutes. Parliament may legislate on matters pending before courts, subject to constitutional convention and political constraint. The United Kingdom constitution is therefore better described as one of separated functions, checking institutions and legally controlled interactions, rather than rigid institutional isolation.

For Part IA purposes, separation of powers connects directly with the previous four weeks. The unwritten and evolving constitution supplies no single master text. Parliamentary sovereignty explains why the courts cannot strike down Acts of Parliament as invalid merely because they breach separation of powers. Constitutional statutes and the reasoning in Thoburn show that some statutory arrangements carry special constitutional weight. The rule of law, especially in Bingham’s account, gives separation of powers its normative edge: public power must be exercised under law, adjudicated by independent courts, and constrained by legality.

The most examinable point is that separation of powers is simultaneously descriptive, doctrinal and normative. Descriptively, it explains the distribution of constitutional labour. Doctrinally, it appears in rules concerning judicial independence, parliamentary privilege, prerogative powers, ouster clauses, contempt, fair trial rights and judicial review. Normatively, it supplies reasons for limiting executive discretion, protecting courts from political pressure, and preserving Parliament’s ability to hold ministers to account. A strong Cambridge answer avoids crude statements such as the United Kingdom has no separation of powers because ministers sit in Parliament. That is true only if separation means complete institutional separation. It is false if separation means an architecture of checks, legality and accountability.

The topic is also a bridge to later public law. Judicial review is the practical mechanism by which the courts police the legal boundaries of executive power. Human rights adjudication raises acute questions about the court’s proper constitutional role. Devolution and retained EU law have complicated the older Diceyan picture. Recent litigation concerning Article 50 and prorogation illustrates the contemporary importance of separation of powers: the courts do not govern, but they may determine whether a purported exercise of governmental power is legally available and compatible with constitutional principle.

The history of separation of powers in England is not the history of a written founding settlement. It is the history of institutional struggle: between Crown and Parliament, between prerogative and statute, and between political authority and legal accountability. The formative idea was not originally a neat tripartite division between legislature, executive and judiciary. It was the proposition that public power must have lawful warrant and that the Crown cannot be judge in its own cause.

The early Stuart conflicts remain central. In the Case of Proclamations, the judges rejected the claim that the King could, by proclamation, change the law or create offences. The case is foundational because it links separation of powers to legality: law-making required legal authority and could not be absorbed into executive will. The political crises of the seventeenth century intensified this theme. The Petition of Right 1628, the Civil War, the Restoration and the Glorious Revolution generated a constitutional order in which the Crown’s powers survived but were increasingly conditioned by statute, parliamentary finance and legal principle.

The Bill of Rights 1688 is therefore a separation of powers instrument, even though it does not use that language. It rejects suspending and dispensing powers without Parliament, condemns taxation without grant of Parliament, and protects parliamentary proceedings from judicial questioning. It does not create a modern liberal constitution. It does, however, allocate constitutional authority: Parliament is the central law-making and fiscal authority; the Crown acts within legally recognised limits; courts do not impeach parliamentary debate. This settlement was reinforced by judicial tenure under the Act of Settlement 1700, which helped to secure the judiciary from direct Crown removal and thus enabled courts to enforce legal limits upon the executive with greater institutional confidence.

Montesquieu’s famous account of the English constitution in The Spirit of the Laws was influential but partly idealised. He admired a system in which liberty was protected because legislative, executive and judicial powers were not concentrated in one person or body. The English constitution he described was already mixed rather than pure. The executive was linked to Parliament; the House of Lords exercised legislative and, historically, judicial functions; and the monarch retained formal powers. The significance of Montesquieu for English constitutional law lies less in literal institutional separation and more in the warning against concentration of unchecked power.

The nineteenth-century constitution developed in a different direction from the American model. Responsible government fused the political executive with the legislature. The Cabinet became dependent on Commons confidence and able to guide legislative business. Dicey’s constitution combined parliamentary sovereignty, the rule of law and constitutional conventions. In that model, the principal control of ministers was political responsibility to Parliament, supplemented by the courts’ insistence that executive action have legal authority. The courts did not review Acts of Parliament, but they did review executive acts, whether purportedly grounded in prerogative, statute or subordinate legislation.

The twentieth and twenty-first centuries altered the balance. The administrative state vastly expanded executive functions. Delegated legislation, regulatory bodies, national security powers and welfare administration all increased the need for legal controls outside ordinary parliamentary debate. The Human Rights Act 1998 gave courts a more visible role in reviewing legislation and public authority action, while formally preserving parliamentary sovereignty. The Constitutional Reform Act 2005 restructured the office of Lord Chancellor, created the Supreme Court of the United Kingdom, and placed judicial independence on an express statutory footing. The modern constitution thus combines historical inheritance with institutional reform: it rejects pure separation, but increasingly articulates the independence and constitutional responsibility of each branch.

The first principle is that separation of powers in the United Kingdom is functional, not absolute. A pure doctrine would require the same persons and bodies not to participate in more than one branch. The British constitution has never operated in that way. Ministers are normally Members of either House of Parliament. The executive initiates most legislation. The courts, through interpretation and common law development, have a creative role. The question is therefore whether the constitution contains sufficient institutional differentiation and checking mechanisms to prevent the arbitrary concentration of power.

The second principle is legality. Executive power must have a lawful source. That source may be statute, prerogative or the common law powers enjoyed by natural persons, but legality requires that the source be identified and that its limits be observed. This is the lesson of the Case of Proclamations and Entick v Carrington. The executive cannot rely upon broad necessity, state convenience or governmental purpose as a substitute for legal authority. In modern terms, judicial review polices this requirement through ultra vires, improper purpose, relevant and irrelevant considerations, procedural fairness, legitimate expectation, proportionality in certain contexts, and common law constitutional rights.

The third principle is parliamentary accountability of the executive. The fusion of Cabinet and Parliament is often treated as a denial of separation of powers. More accurately, it is the mechanism through which the executive is politically accountable. Ministers answer questions, defend policy, justify expenditure, and may lose office if they lose the confidence of the Commons. This is not a legal substitute for judicial review, but it is a constitutional control. A sophisticated answer distinguishes accountability to Parliament from control by Parliament. In practice, a government with a stable majority can dominate much of the Commons agenda. Select committees, opposition days, urgent questions, backbench rebellions and the House of Lords provide checks, but they are uneven and often political rather than legal.

The leading cases show how separation of powers is enforced through ordinary legal reasoning rather than through a freestanding constitutional code. They also show that the doctrine protects different institutions in different contexts.

The Case of Proclamations is the starting point for executive legality. The judges denied that the King could create new legal obligations by proclamation. The point is not simply historical. It remains the basis for the modern rule that prerogative power cannot change domestic law or override statute. Executive power must be traced to law.

Entick v Carrington is the classic civil liberties case. State officers entered Entick’s premises and seized papers under a warrant issued by the Secretary of State. The court held the search unlawful because no legal authority justified it. The separation principle is plain: executive officials cannot create coercive power for themselves. If they interfere with liberty or property, they must show legal warrant.

M v Home Office is a modern affirmation that ministers are subject to court orders. The Home Secretary was held capable of being in contempt after an asylum seeker was removed contrary to an undertaking and order. The case matters because it rejects any residual idea that the executive obeys law merely as grace. Courts may make coercive orders against ministers acting in their official capacity.

Fire Brigades Union concerns the relationship between prerogative, statute and parliamentary intention. Parliament had enacted a statutory criminal injuries compensation scheme, but the relevant provisions had not been brought into force. The minister introduced a different tariff scheme under prerogative power. A majority held that this was unlawful because the minister had used prerogative power in a way that frustrated the statutory scheme. The decision illustrates the subordination of prerogative to statute and the courts’ willingness to police executive attempts to bypass Parliament.

Anderson illustrates the judicial function in sentencing. The Home Secretary’s power to set the tariff for mandatory life sentence prisoners was held incompatible with Article 6 of the European Convention on Human Rights. Although the decision arose under the Human Rights Act, its deeper constitutional significance lies in the distinction between executive administration of punishment and judicial determination of sentence.

Jackson is principally a parliamentary sovereignty case, but it matters for separation of powers because several speeches connect legislative authority, the rule of law and the judicial role. The case concerned the validity of the Hunting Act 2004 under the Parliament Acts procedure. The House of Lords upheld the Act. Some dicta suggested that the courts might react differently to legislation abolishing judicial review or undermining the rule of law. Those dicta are not a licence to disregard Acts of Parliament, but they are important evidence of judicial anxiety about ultimate constitutional fundamentals.

Miller No 1 and Miller/Cherry are the contemporary centrepieces. In Miller No 1, the Government could not trigger Article 50 using prerogative power because doing so would alter domestic law and affect statutory rights. Parliament was required. In Miller/Cherry, the advice to prorogue Parliament was unlawful because it frustrated Parliament’s constitutional functions without reasonable justification. The cases are controversial, but they are doctrinally coherent if read as applications of legality, parliamentary accountability and the judicial duty to determine the limits of public power. They do not convert courts into political actors; they insist that even high constitutional powers have legal boundaries.

See practice questions section below.

See further reading section below.