Meaning, Formation and Evidence
Customary International Law (CIL) consists of unwritten international legal rules arising from a general practice accepted as law. It develops through the conduct and legal conviction of States rather than primarily through a negotiated treaty.
Article 38(1)(b) of the Statute of the International Court of Justice (ICJ) identifies “international custom, as evidence of a general practice accepted as law” as a source of international law.
Two elements must ordinarily coexist:
- State Practice: sufficiently widespread, representative and consistent conduct of States. It may include legislation, executive actions, diplomatic correspondence, military manuals, treaty practice, positions in international organisations and conduct before international courts.
- Opinio Juris: States follow the practice because they consider themselves legally required or legally entitled to do so, rather than merely acting from courtesy, convenience, political preference or habit.
Practice need not be absolutely universal or perfectly uniform. Particular importance may attach to the practice of States specially affected by the subject matter.
Evidence of custom may therefore emerge from national laws, diplomatic statements, UN resolutions, official legal positions, judicial decisions and consistent patterns of State behaviour. UN General Assembly resolutions do not automatically create customary law, but may provide evidence of opinio juris or contribute to the development of a customary rule.
Types, Treaties and Important Doctrines
Custom may operate at different levels:
- General custom: sufficiently widespread practice capable of binding the international community of States generally.
- Regional custom: develops among States of a particular region.
- Local or bilateral custom: may arise between a small number of States where consistent practice and acceptance as law are established.
In Asylum Case (Colombia v. Peru, 1950), the ICJ emphasised that a party relying on regional custom must establish constant and uniform usage accepted as law.
The North Sea Continental Shelf Cases (1969) became foundational for distinguishing State practice from opinio juris. They also demonstrated that a treaty rule can, under appropriate conditions, contribute to the emergence of a customary rule where subsequent practice becomes sufficiently extensive and representative and is accompanied by opinio juris.
Treaties and custom therefore interact in several ways:
- a treaty may codify an already existing customary rule;
- treaty practice may contribute to the development of new custom;
- a customary rule may continue to bind States independently of a treaty.
Hence, even a State that is not party to a particular treaty may still be bound by a rule contained in it if that rule independently constitutes customary international law.
An important exception is the persistent objector doctrine: a State that clearly and consistently objects to an emerging customary rule while it is being formed may, under limited circumstances, avoid being bound by it. This doctrine cannot operate against peremptory norms (jus cogens).
Custom, Jus Cogens and Contemporary Significance
Customary international law must be distinguished from jus cogens.
- Customary rule: generally binding because of State practice plus opinio juris.
- Jus cogens: a peremptory norm recognised by the international community of States as a whole from which no derogation is permitted.
Prohibitions against genocide, slavery and torture, for example, are widely recognised as possessing peremptory status.
Customary international law is particularly significant where treaty law is absent, incomplete or does not bind all relevant States. Major fields shaped substantially by custom include:
- State immunity and diplomatic relations;
- use of force and self-defence;
- international humanitarian law;
- State responsibility;
- law of the sea;
- treatment of foreign nationals;
- certain environmental obligations.
In the Nicaragua v. United States (1986) case, the ICJ demonstrated that customary rules concerning the prohibition on the use of force and non-intervention could operate independently of parallel treaty obligations.
For India, customary international law is relevant to maritime rights, diplomatic relations, State immunity, armed conflict and international environmental obligations. Indian courts may refer to international custom, particularly where domestic legislation is silent, but international rules cannot override contrary Parliamentary legislation or the Constitution.
Customary international law is therefore crucial because it enables international law to develop without requiring every State to ratify a new treaty, while its demanding requirements of State practice and opinio juris prevent every recurring diplomatic practice from automatically becoming legally binding.
