Public international law is the body of legal rules, norms, and standards that apply between sovereign states and other entities that are legally recognized as international actors. Unlike domestic law, which is typically created by a legislature, international law operates in a decentralised system where states themselves are the primary lawmakers and subjects. This guide explores the foundational principles of international law, its primary sources, and its complex and ever-evolving relationship with the domestic law of the United Kingdom.
💡 Key Takeaway
International law is fundamentally based on state consent. States agree to be bound by treaties and their consistent practice evolves into customary rules. The system's effectiveness relies on the principles of reciprocity, good faith (pacta sunt servanda), and the political will of states to uphold their international obligations, even when they conflict with domestic policy.
The Sources of International Law
Article 38(1) of the Statute of the International Court of Justice (ICJ) is the universally recognised authoritative starting point for identifying the sources of international law. It outlines the primary and subsidiary sources that the Court shall apply when deciding disputes between states.
Primary Sources: Treaties and Custom
The two most important sources are treaties (also known as conventions, pacts, or agreements) and customary international law. Treaties are formal written agreements between states that create binding legal obligations for the parties. The Vienna Convention on the Law of Treaties 1969, largely considered a codification of customary law, governs their formation, interpretation, and termination. A key UK case on treaty interpretation is Fothergill v Monarch Airlines Ltd [1981] AC 251, which considered the use of preparatory works (travaux préparatoires) in understanding treaty terms.
Customary international law (CIL) is unwritten law that derives from the consistent and widespread practice of states followed from a sense of legal obligation, known as opinio juris sive necessitatis. A classic authority is the North Sea Continental Shelf Cases (Germany/Denmark; Germany/Netherlands) [1969] ICJ Rep 3. For a rule to become custom, the practice must be extensive and virtually uniform, as affirmed in the Asylum Case (Colombia/Peru) [1950] ICJ Rep 266.
Subsidiary Sources: General Principles and Judicial Decisions
General principles of law recognized by 'civilized nations' (a term now interpreted to mean all major legal systems) are used to fill gaps in treaty and customary law. These include principles like estoppel, good faith, and res judicata. Judicial decisions, from both international courts like the ICJ and domestic courts, are a subsidiary means for determining rules of law. While there is no formal rule of binding precedent (stare decisis) in international law, the ICJ's decisions are highly persuasive and it rarely departs from its own previous findings. The writings of highly qualified publicists are also mentioned in Article 38 as a subsidiary source.
State Sovereignty and Jurisdiction
The principle of state sovereignty is the cornerstone of the international legal system. It means that each state is independent, equal, and has exclusive control over its internal affairs, territory, and population. This principle is a bedrock of the UN system, enshrined in Article 2(1) of the UN Charter.
In the Island of Palmas Case (Netherlands/USA) (1928) 2 RIAA 829, the arbitrator Max Huber provided a classic definition: "Sovereignty in the relations between States signifies independence. Independence in regard to a portion of the globe is the right to exercise therein, to the exclusion of any other State, the functions of a State."
However, sovereignty is not absolute. A state's jurisdiction—its power to make, apply, and enforce its laws—is limited by the rights of other states under international law. For example, the doctrine of state immunity, a rule of CIL codified in the UK by the State Immunity Act 1978, generally prevents one state from being sued in the courts of another. This was central in Jones v Ministry of Interior of Saudi Arabia [2006] UKHL 26, where the House of Lords upheld Saudi Arabia's immunity from a civil claim for torture, finding the statutory immunity granted by the 1978 Act was not overridden by the international prohibition on torture (a jus cogens norm).
⚠️ Common Mistake
A common error is to confuse the government with the state. The 'state' is the permanent legal entity in international law, possessing rights and obligations. The 'government' is the temporary political agent of the state. A change in government, even through unconstitutional means, does not affect the state's continuing legal personality or its obligations under international law.
International Law and UK Domestic Law
The relationship between international and domestic law is complex and is traditionally explained by two main theories: monism and dualism. Monists see international and domestic law as part of a single legal order, whereas dualists see them as two separate systems. The UK traditionally follows a dualist approach, meaning international law has no direct effect in UK law until it is deliberately incorporated by an Act of Parliament or recognised by the common law.
Incorporation of Treaties vs. Customary Law
For a treaty to become part of UK law, Parliament must pass legislation giving it effect. A prime example is the Human Rights Act 1998, which incorporated most rights from the European Convention on Human Rights into UK domestic law. Without such an Act, a treaty is not a source of rights or obligations that can be enforced by individuals in UK courts, as established in Maclaine Watson & Co Ltd v Department of Trade and Industry [1989] Ch 72.
Customary international law, by contrast, is generally treated as part of the common law and can be applied directly by UK courts, provided it is not inconsistent with statute or a binding judicial precedent. This was famously articulated by Lord Denning in Trendtex Trading Corp v Central Bank of Nigeria [1977] QB 529. However, the supremacy of Parliament means that a clear statute will always prevail over a conflicting rule of CIL, as seen in R v Lyons [2002] UKHL 44.
| Source of International Law | Application in UK Law | Key Principle / Case |
|---|---|---|
| Treaties | Requires incorporation by an Act of Parliament. | Dualism (Maclaine Watson v DTI) |
| Customary International Law | Applied directly by courts as part of the common law. | Incorporation (Trendtex Trading v CBN) |
| Conflict with UK Statute | UK statute always prevails. | Parliamentary Sovereignty (R v Lyons) |
The Role of International Institutions
Key institutions underpin the international legal order. The United Nations (UN) is the principal global organisation for cooperation. Its Security Council has the power to make decisions binding on all member states under Chapter VII of the UN Charter to maintain international peace and security. The International Court of Justice (ICJ) is the UN's principal judicial organ, settling legal disputes between states that consent to its jurisdiction. Its judgment in the Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America) [1986] ICJ Rep 14 is a landmark decision on the customary law prohibitions on the use of force and intervention.
📝 Exam Tip
When analysing a problem question involving international law and the UK, always determine the source of the alleged rule first. Is it from a treaty or custom? If it's a treaty, have you checked if it has been incorporated into UK law by an Act of Parliament? This is a crucial first step that is often missed. For CIL, consider whether it conflicts with any UK statute. Precision on the source and status of the rule is key to a first-class answer.
State Responsibility
When a state breaches an international obligation, it incurs state responsibility. The core principles are now authoritatively codified in the International Law Commission's (ILC) Articles on Responsibility of States for Internationally Wrongful Acts (2001). A wrongful act requires two elements: attribution (the conduct must be attributable to the state under international law) and breach (the conduct must constitute a breach of an international obligation). The case of Corfu Channel Case (United Kingdom v Albania) [1949] ICJ Rep 4 established that a state is responsible for acts occurring on its territory of which it has, or ought to have, knowledge.
Frequently Asked Questions (FAQ)
Is international law really 'law'?
Yes. While it lacks a central enforcement body like a global police force, it is considered law because states and other international actors treat it as such, citing it in their disputes and in their courts. It is created through recognised legal processes (treaties, custom) and breaches have legal consequences, including obligations to make reparations and the possibility of sanctions. The House of Lords in R v Jones (Margaret) [2006] UKHL 16 extensively discussed the legal status of the crime of aggression in international law before concluding it was not yet a crime in UK domestic law.
What is a 'jus cogens' norm?
A jus cogens or peremptory norm is a fundamental, overriding principle of international law, accepted by the international community of states as a norm from which no derogation is ever permitted. Examples include the prohibitions on genocide, slavery, torture, and wars of aggression. A treaty that conflicts with a jus cogens norm is void under Article 53 of the Vienna Convention on the Law of Treaties.
Can individuals be subjects of international law?
Traditionally, only states were subjects of international law. However, this has evolved significantly since 1945. Individuals now have rights under international human rights law (e.g., the ECHR) and can be held directly responsible for international crimes (e.g., genocide, war crimes) before tribunals like the International Criminal Court (ICC).
What is the difference between the ICJ and the ICC?
The International Court of Justice (ICJ), or 'World Court', is a civil court that hears disputes between states. Its jurisdiction is based on the consent of the states involved. The International Criminal Court (ICC) is a criminal court that prosecutes individuals for the most serious international crimes: genocide, crimes against humanity, and war crimes. The UK is a party to the statutes of both courts.
What is the legal effect of a UN General Assembly resolution?
UN General Assembly resolutions are generally not legally binding. They are recommendations. However, they can have significant legal weight. They can be evidence of existing customary international law, or they can contribute to the formation of new CIL if they are widely supported by states and reflect their sense of legal obligation (opinio juris).