Separation of Powers in the UK

Introduction

The separation of powers is a constitutional idea that divides state power between three branches so no single body holds all authority. In the UK, this division is real but not rigid. Ministers sit in Parliament, the Government proposes most legislation, and the courts cannot strike down Acts of Parliament. Even so, each branch has a distinct role, and there are meaningful checks to keep power in balance.

Parliament makes and scrutinises laws. The Government runs the country, using statutory powers and certain prerogative powers. The courts interpret the law and review the lawfulness of executive action. Since the Human Rights Act 1998 (HRA), courts can also assess whether legislation is compatible with Convention rights (issuing declarations of incompatibility where needed). Modern case law—from Entick v Carrington to the Miller cases—shows how these checks apply in practice.

What You'll Learn

• How Parliament, Government and the courts share and limit power in the UK
• The practical effect of parliamentary sovereignty and political accountability
• When prerogative powers can be used and how statute takes priority
• How judicial review works, including grounds and limits
• The role of the Human Rights Act 1998 (ss 3, 4 and 6)
• Key cases on separation of powers, including De Keyser, Fire Brigades Union, Miller I and Miller II
• How to structure answers to constitutional and judicial review problems

Core Concepts

Parliament and sovereignty

• Role: The UK Parliament (Commons and Lords) enacts primary legislation. Under the Parliament Acts 1911 and 1949, the Commons can, in limited circumstances, legislate without the Lords’ consent.
• Sovereignty: Courts give effect to Acts of Parliament. In Pickin v British Railways Board [1974], the House of Lords confirmed the “enrolled bill” rule: courts do not question how an Act was passed once it has received Royal Assent.
• Human rights: The HRA 1998 preserves Parliament’s position. Courts use s3 to interpret legislation compatibly with Convention rights where possible; where not possible, they may issue a s4 declaration of incompatibility. A declaration does not invalidate the Act but flags the issue for Parliament to address.

Delegated legislation and accountability

• Delegation: Parliament often delegates detail to ministers via enabling Acts. Ministers then make statutory instruments (SIs), subject to procedures such as affirmative or negative resolution.
• Control: Delegated legislation must stay within the scope of the enabling Act. It can be quashed on grounds of illegality, procedural impropriety, or irrationality. Parliamentary committees also scrutinise SIs.
• Judicial review: Delegated legislation is reviewable in the same way as other executive acts. Ouster clauses and broad discretions are read carefully; the courts ensure that ministers stay within the legal limits Parliament set.

Executive power and the royal prerogative

• Executive role: The Government (Prime Minister, ministers, and the civil service) runs public administration, proposes legislation, and sets policy. It also uses prerogative powers: treaty-making, defence, passports, and certain appointments.
• Reviewability: In CCSU v Minister for the Civil Service [1985], the House of Lords confirmed that prerogative powers are reviewable where the subject matter is justiciable.
• Statute over prerogative: Attorney-General v De Keyser’s Royal Hotel [1920] establishes that where statute covers the same ground as a prerogative, the statute prevails and the prerogative cannot be used to bypass it.
• Modern limits: In R (Miller) v Secretary of State for Exiting the EU [2017] (Miller I), the Supreme Court held the Government could not use the prerogative to trigger Article 50 because it would alter domestic law and rights without Parliament. In R (Miller) v Prime Minister [2019] (Miller II/Cherry), the prorogation of Parliament was unlawful because it frustrated Parliament’s ability to legislate and scrutinise.

The courts and judicial review

• Role: The courts interpret and apply the law, and ensure public bodies act within their legal powers. The Constitutional Reform Act 2005 strengthened judicial independence and created the Supreme Court (which began work in 2009).
• Grounds: CCSU identified three classic grounds for judicial review—illegality, irrationality (Wednesbury unreasonableness), and procedural impropriety. Proportionality is applied where Convention rights are engaged.
• Human rights: Public authorities must act compatibly with Convention rights (s6 HRA 1998). Courts use s3 interpretive techniques where possible; if not, they may issue a s4 declaration (e.g., R (Anderson) v Secretary of State for the Home Department [2002]).
• Deference and limits: Courts do not rule on political merits. In national security and foreign affairs, the courts may accept a wider margin of judgment, but a legal error remains reviewable (e.g., R (Lord Carlile) v Home Secretary [2014]).

Key Examples or Case Studies

Entick v Carrington (1765)

• Key point: Officials must point to lawful authority for any intrusion on liberty or property.
• Application: A basic rule that strengthens judicial control over executive action.

Attorney-General v De Keyser’s Royal Hotel [1920] AC 508

• Key point: If Parliament has legislated, ministers cannot use prerogative powers to avoid statutory limits.
• Application: Always check for relevant statutes before relying on the prerogative.

R v Secretary of State for the Home Department, ex parte Fire Brigades Union [1995] 2 AC 513

• Key point: The Home Secretary acted unlawfully by seeking to use prerogative powers to introduce a scheme inconsistent with a statutory scheme that Parliament had enacted.
• Application: Executive cannot use prerogative powers to sidestep or frustrate Parliament’s choices.

CCSU v Minister for the Civil Service [1985] AC 374 (the GCHQ case)

• Key point: Prerogative powers are subject to judicial review where the issue is justiciable; various grounds of review apply.
• Application: Don’t assume prerogative equals immunity from review.

R (Anderson) v Secretary of State for the Home Department [2002] UKHL 46

• Key point: A minister setting tariffs for mandatory life sentences breached Article 6 ECHR. The court issued a declaration of incompatibility.
• Application: Article 6 requires an independent and impartial tribunal for sentencing decisions.

R (on the application of Miller) v Secretary of State for Exiting the EU [2017] UKSC 5 (Miller I)

• Key point: Triggering Article 50 required an Act of Parliament; prerogative could not change domestic rights.
• Application: Check whether the proposed executive action would alter domestic law or rights.

R (Miller) v Prime Minister; Cherry v Advocate General for Scotland [2019] UKSC 41 (Miller II)

• Key point: Prorogation was justiciable and unlawful where it frustrated or prevented Parliament from carrying out its constitutional role without reasonable justification.
• Application: Executive actions that disable parliamentary scrutiny can be unlawful.

R v Chaytor and others [2010] UKSC 52

• Key point: Parliamentary privilege does not cover criminal acts such as false expenses claims.
• Application: Privilege protects proper parliamentary proceedings, not criminal conduct.

R (Lord Carlile) v Home Secretary [2014] UKSC 60

• Key point: In national security, the courts may allow a wider margin of judgment, but the decision must still be lawful and rational.
• Application: Deference does not mean abdication of review.

R (on the application of Bancoult) v Secretary of State for Foreign and Commonwealth Affairs (No 2) [2008] UKHL 61

• Key point: Orders in Council made under the prerogative are reviewable; principles of fairness and abuse of power apply, though the challenge failed on the facts.
• Application: Prerogative legislation is not beyond legal control.

R (Quila) v Secretary of State for the Home Department [2011] UKSC 45

• Key point: A policy setting a minimum age of 21 for spousal visas was a disproportionate interference with Article 8.
• Application: Proportionality analysis is central where Convention rights are engaged.

R (Gentle) v Prime Minister [2008] UKHL 20

• Key point: Article 2 did not impose a duty to hold an inquiry into the decision to go to war; however, legality of the decision-making process is reviewable.
• Application: Courts assess lawfulness even in sensitive areas, without ruling on political wisdom.

Practical Applications

• Identify the actor and the power

  • Who acted (minister, department, local authority, or the Prime Minister)?
  • What is the source of power—statute, delegated legislation, or prerogative?

• Check for statutory coverage and primacy

  • If there is an Act on point, it governs. The prerogative cannot be used to bypass statute (De Keyser; Fire Brigades Union).
  • For delegated legislation, confirm the enabling Act permits the measure and that the correct parliamentary procedure was followed.

• Choose the right public law challenge

  • Grounds: illegality (including acting for an improper purpose or fettering discretion), irrationality, procedural impropriety, legitimate expectation, and proportionality where rights apply.
  • Evidence: assemble the decision, the reasons (if any), consultation record, and relevant guidance or codes.

• Apply the Human Rights Act 1998 where relevant

  • Is a Convention right engaged? If yes, consider proportionality and s6 duties on public authorities.
  • s3 interpretation first; if the words cannot bear a compatible meaning, consider a s4 declaration of incompatibility.
  • Remedies: quashing order, mandatory order, injunction, declaration, and in HRA cases, damages in limited circumstances.

• Address justiciability and deference

  • Some issues (e.g., allocation of parliamentary time) are non-justiciable; others (e.g., prorogation that disables scrutiny) are justiciable (Miller II).
  • In national security or foreign policy, expect a wider margin of judgment, but legal limits still apply (Lord Carlile).

• Remember parliamentary sovereignty and privilege

  • Courts will not strike down Acts or question how an Act was passed (Pickin).
  • Privilege protects parliamentary proceedings, but not criminal conduct (Chaytor).

• Note current constitutional context

  • Post-Brexit, changes under the European Union (Withdrawal) Act 2018 and the Retained EU Law (Revocation and Reform) Act 2023 affect the status of retained EU law. This does not alter the basic separation of powers, but it may affect which standards and rights apply in a given case.

Summary Checklist

• Parliament makes primary law; ministers make SIs within statutory limits
• Parliamentary sovereignty: courts apply Acts and do not question proceedings
• Statute outranks prerogative where both cover the same subject (De Keyser)
• Executive action, including under the prerogative, is reviewable (CCSU)
• Use JR grounds: illegality, irrationality, procedural impropriety; add proportionality for rights cases
• HRA 1998: s3 interpretation, s4 declaration, s6 duty on public authorities
• Courts do not judge political merits; they rule on legality and process
• National security and foreign policy attract a wider margin, but errors of law remain reviewable
• Privilege protects proceedings, not crimes (Chaytor)
• Prorogation and major constitutional steps require legal authority and respect for Parliament’s role (Miller I and II)

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