Separation of powers

Separation of powers is often misunderstood in first-year constitutional law because students approach it as if the United Kingdom must either possess, or lack, the doctrine in a pure form. That is the wrong question. The United Kingdom has never adopted a rigid Montesquieuian division between legislature, executive and judiciary. Ministers sit in Parliament. The executive normally controls the Commons timetable. Parliament may legislate on almost any subject. The courts do not strike down Acts of Parliament for unconstitutionality in the ordinary American sense. Yet it would be equally mistaken to conclude that separation of powers is absent. It operates as a constitutional principle of institutional discipline: those who make law, execute law and adjudicate disputes should not collapse into a single unchecked authority.

For Durham purposes, this week should be read as the bridge between the earlier materials. Week 1 established that the UK constitution is plural, historical and partly legal, partly conventional. Week 2 examined sovereignty and the legislative supremacy of Parliament. Week 3 introduced constitutional statutes and the limits of implied repeal. Week 4 gave the rule of law as a controlling constitutional value. Separation of powers now asks how those materials are institutionalised. If Parliament is sovereign, who controls the executive? If courts cannot invalidate primary legislation, what is judicial review for? If ministers sit in Parliament, where is the separation? If judges are independent, how far may they scrutinise high policy?

The answer is functional rather than formal. Parliament authorises and scrutinises the executive; the executive proposes policy and administers statutory powers; the courts determine legal rights and police legality. Those functions overlap, but overlap is not identical to confusion. The constitution tolerates overlap where it promotes responsible government; it resists overlap where it threatens legality, impartial adjudication, or democratic accountability. A minister may be an MP; that is constitutionally orthodox. A minister may not decide the sentence of an individual prisoner after conviction; that offends the judicial function. Parliament may legislate retrospectively; but if it directs the outcome of pending litigation in favour of the state, serious separation concerns arise. A court may interpret statute robustly; but it must not convert disagreement about policy into a merits appeal.

The practical importance is that separation of powers supplies both a language of constitutional evaluation and a legal premise in cases. It underlies judicial independence under the Constitutional Reform Act 2005, parliamentary privilege under Article 9 of the Bill of Rights 1688, the law of contempt, the scope of judicial review, the treatment of ouster clauses, the limits of prerogative power, and the judicial insistence that the executive must obey court orders. It is also central to recent constitutional controversy: prorogation, delegated legislation, Henry VIII clauses, ministerial criticism of judges, and attempts to insulate administrative bodies from review.

In an examination, do not write a descriptive essay listing the three branches. The better answer identifies the particular institutional risk: executive domination of Parliament, legislative intrusion into adjudication, judicial overreach into policy, or executive resistance to law. Then explain which constitutional mechanisms respond to that risk. Separation of powers is not a diagram. It is an argument about constitutional settlement, institutional competence and lawful authority.

The British constitution developed before modern constitutional theory supplied a neat vocabulary. Medieval and early modern government did not divide power according to the categories now called legislative, executive and judicial. The Crown governed through prerogative, council, judges and Parliament; offices and functions were mixed. The common law courts nevertheless developed an important constitutional idea: public power required legal authority. The Case of Proclamations and Entick v Carrington are not modern separation-of-powers cases in form, but they are foundational in substance. They reject the proposition that executive command can create law or invade liberty without lawful warrant.

The seventeenth-century conflicts between Crown and Parliament shifted the centre of legislative authority. The Bill of Rights 1688 condemned suspending and dispensing powers exercised by the Crown without Parliament. Article 9 protected parliamentary proceedings from external challenge. That settlement did not produce a codified separation of powers. Rather, it produced two enduring propositions: first, executive power is legally limited; secondly, Parliament is the central law-making institution. Those propositions later sat alongside the doctrine of responsible government, under which ministers are politically accountable to Parliament and must normally command the confidence of the House of Commons.

Montesquieu’s account of the English constitution in De l’esprit des lois famously influenced separation-of-powers thought, particularly in the United States. But his account was partly idealised. Eighteenth-century England did not maintain hermetically sealed branches. The Lord Chancellor was simultaneously a Cabinet minister, presiding officer of the House of Lords, and senior judge. The House of Lords exercised legislative and judicial functions. Ministers sat in Parliament. The executive was drawn from the legislature. Dicey later placed parliamentary sovereignty and the rule of law at the centre of British constitutionalism, but he did not present a rigid separation of powers as an autonomous master doctrine.

The nineteenth and twentieth centuries deepened executive power. The growth of the administrative state meant that ministers and departments made policy, administered schemes, exercised discretionary powers and promoted legislation. Parliamentary government became, in practice, government through a disciplined majority in the Commons. This produced the familiar modern anxiety: if the executive usually controls the House of Commons, the formal supremacy of Parliament may conceal executive predominance. Delegated legislation, statutory instruments, framework Acts and Henry VIII clauses intensify that concern because they permit ministers to make or amend law with limited parliamentary scrutiny.

The judiciary’s institutional position changed more slowly but decisively. Judicial review expanded during the twentieth century from narrow control of jurisdictional error towards a more general supervisory jurisdiction over legality, fairness and rationality. The Human Rights Act 1998 then required courts to review public authority action for compatibility with Convention rights and to interpret legislation compatibly where possible. The Constitutional Reform Act 2005 completed a major institutional reform by altering the office of Lord Chancellor, creating the UK Supreme Court, and putting the defence of judicial independence into statutory form. These reforms were not cosmetic. They clarified that final adjudication should not be carried out by a committee of the legislature and that ministers must not use special access to influence particular judicial decisions.

The modern doctrine is therefore historically layered. It contains a monarchical legacy, a parliamentary settlement, responsible government, administrative legality, judicial independence and rights review. It is not reducible to American-style checks and balances. Nor is it merely a convention. It is an organising principle recognised by courts and statutes, operating differently in different contexts. Its deepest historical lesson is that concentration of power is constitutionally dangerous, especially when executive convenience is presented as constitutional necessity.

The first principle is functional separation. The constitution distinguishes law-making, administration and adjudication, even though institutions may overlap. Parliament legislates; ministers govern under statute and prerogative; courts decide disputes according to law. The key question is whether an institution is performing a function in a constitutionally legitimate way. It is not objectionable that the executive initiates most legislation. It is objectionable if the executive claims to alter legal rights without parliamentary authority, or to determine the legal consequences of its own unlawful conduct.

The second principle is legality of executive action. The executive has no general dispensing power. It acts through statutory powers, common law powers, prerogative powers and contractual or proprietary capacities, but those powers are legally bounded. The courts decide whether a power exists, whether it has been used for a proper purpose, whether relevant considerations were taken into account, and whether procedural fairness was observed. This is where separation of powers intersects with the rule of law. Judicial review is not a judicial seizure of executive government; it is the legal mechanism by which executive government is kept within its authority.

The third principle is parliamentary accountability. In the orthodox model, ministers are accountable to Parliament for policy and administration. Questions, select committees, debates, opposition days, supply, legislation and confidence are political controls. They are constitutionally indispensable because many executive choices are not suitable for judicial merits review. But they are also imperfect. A government with a stable majority can often manage parliamentary scrutiny. Durham essays should therefore avoid romantic language about Parliament as if it were always an external check on the executive. The Commons may check government; it may also be controlled by it. Separation of powers in the UK must be analysed against party discipline and executive dominance.

The case law should be learned as a sequence rather than a list. The earliest authorities establish that executive power is legally limited. The Case of Proclamations denied the Crown a unilateral law-making power. Entick v Carrington denied executive officials a power to invade property and liberty without legal authority. These cases remain doctrinally alive because they express the first proposition of separation of powers in the UK: government is not the source of its own authority.

A second line of cases concerns the relation between courts and Parliament. Courts respect parliamentary privilege and will not question proceedings in Parliament, but they interpret and apply statutes as law. Pickin v British Railways Board illustrates the enrolled Act principle: courts will not investigate the internal parliamentary process by which an Act was passed. Jackson v Attorney General is more complex. The House of Lords upheld the validity of the Hunting Act 2004, but several speeches suggested that parliamentary sovereignty is now understood against the background of legality and constitutional principle. Jackson should not be cited for the crude proposition that courts may simply disapply Acts of Parliament. Its importance lies in its constitutional tone: the relationship between sovereignty, rule of law and courts is not exhausted by nineteenth-century formulations.

A third line concerns the judicial function. In R v Secretary of State for the Home Department, ex p Anderson, the House of Lords held that the Home Secretary’s power to set mandatory life sentence tariffs was incompatible with Article 6. The decision turned on the separation between sentencing, which is judicial, and executive policy. Anderson is a valuable case because it shows separation of powers producing a concrete result: even an elected minister cannot perform a function that properly belongs to an independent court.

A fourth line concerns executive obedience to courts. M v Home Office is indispensable. The House of Lords held that a minister could be found in contempt of court and that the Crown was not above injunctive orders. The case gives separation of powers a hard legal edge. Courts do not merely advise the executive; their orders bind it. The rule of law would be emptied if ministers treated court orders as optional.

A fifth line concerns constitutional fundamentals and justiciability. A v Secretary of State for the Home Department demonstrates that even emergency and national security measures are legally reviewable. Miller/Cherry goes further in contemporary terms: the Supreme Court held that advice to prorogue Parliament was justiciable and unlawful because it frustrated Parliament’s constitutional functions without reasonable justification. The case is a leading illustration of the courts protecting the institutional conditions of parliamentary accountability, not substituting themselves for Parliament.

Finally, the ouster clause cases, especially Privacy International, show courts defending the supervisory jurisdiction. Parliament may use clear language, but the courts approach exclusions of review with constitutional suspicion. This is not judicial vanity. It reflects the idea that legality requires an independent institution capable of determining whether public power has exceeded its limits.

See practice questions section below.

See further reading section below.