Customary International Law

Customary international law consists of unwritten rules derived from a general practice of states that is accepted as legally obligatory. It can bind a state without treaty ratification because its authority arises from custom, not from consent to the treaty containing a similar rule.

Requirements for a binding custom

Under Article 38(1)(b) of the ICJ Statute, a customary rule requires two elements: sufficiently general and consistent state practice, and acceptance of that practice as law, called opinio juris.

• Practice may include diplomatic conduct, legislation, official statements, military manuals and conduct before international organisations.
• No fixed period is essential, but the practice must be sufficiently widespread and representative; inconsistent conduct can prevent a rule from emerging.

Relationship between treaties and custom

Under Article 34 of the Vienna Convention on the Law of Treaties, a treaty does not create obligations for a third state without its consent. However, Article 38 recognises that a rule stated in a treaty may bind a third state when that rule has independently become customary international law.

• A treaty may codify an already existing custom, crystallise an emerging custom, or contribute to the later formation of a new custom.
• The non-party state is then bound by the customary rule in its customary capacity, not by the treaty itself.
• Treaty text and widespread participation may provide evidence of custom, but they do not automatically establish state practice and opinio juris.

Limits and exceptions

A state may avoid an emerging customary rule if it acts as a persistent objector, clearly and consistently opposing the rule while it is being formed. Mere failure to ratify a treaty is not sufficient objection.

• Objection raised only after the customary rule has formed does not ordinarily exempt the state.
• No persistent objection can permit derogation from a jus cogens norm, which is a peremptory norm accepted and recognised by the international community of states as a whole.