• Customary international law refers to international obligations arising from established international practices.
  • Such International Practice results from a general and consistent practice of states that they follow from a sense of legal obligation.
  • For example- doctrine of non-refoulement and the granting of immunity for visiting heads of state.

Definition According to ICJ Statute

According to the language of article 38 (1) (b) formation of customary international law requires two basic element, i.e.

  1. State practice and
  2. Opinio Juris Sive Necessitatis



Customary rules of international law have developed as a result of-:

  • Diplomatic relations between states
  • Practice of international organs
  • State laws, decisions of state courts
  • Treaties between states

State Practice

  • State practice is an objective requirement that focuses on the behavior of states. It is also called as the material content of Customary International Law.
  • It is the actual behaviour of the states. State practice , covers any act or statements by a state from which views about customary law may be inferred. However, every practice does not have an equal weight.
  • Examples of the state practice -: administrative acts of states; legislations; decision of court; memoirs of past leaders; diplomatic interchanges etc.

How much State Practice is required ?

  • In Nicaragua v. USA case, court held that it was not necessary that the practice in question had to be in absolutely rigorous conformity with the purported customary law.

Other important cases

  • In Construction of Wall Case, the international court has also noted that evidence of the existence of rules and principles may be found in resolutions adopted by the Gen assembly and the Security Council of the United Nations.
  • State municipal laws may in certain circumstances formed the basis of customary rules. In Scotia case decided by the US Supreme Court in 1871, a British ship has sunk an American vessel on the high seas. The court held that British navigational procedure established by an act of Parliament formed the basis of the relevant international custom since other state has legislated in one surely identical terms.
  • In the Advisory Opinion on the Use of Nuclear Weapon case court held that no rule can be created on opinion juris without state practice.

Opinio Juris

  • The opinio juris requirement focuses on the subjective belief of the state in question and requires that a state believe itself to be bound by the customary rule in question.
  • It is the belief that the state activity legally obligated as distinguished from motives of fairness, convenience or morality. State will behave in a certain way because they are convinced it is binding upon them to do so.
  • The PCIJ has expressed this point in Lotus Case.

Fact – Collision of two ships named Lotus (French ship) and Boz-Kourt (Turkish ship) happened on high seas. Several people died and Turkish ship had sunk. When Lotus reached Istanbul, the French Officer arrested of manslaughter.

Issue- Whether Turkey had Jurisdiction to try this case or not ?

Argument by France- France held that there existed a customary rule that only Flag State has the Jurisdiction. To justify this, France referred to the absence of previous criminal prosecution by such states in similar situations and from this deduced tacit consent in the practice which therefore became a legal custom.

Decision – The Court rejected rejected this argument and declared abstention from instituting criminal proceedings could be proved in fact, it would not amount to a custom. Court further held that held that only if such abstention were based on their (the states) being conscious of a duty to abstain would it be possible to speak of an international custom. Thus the essential ingredient of obligation was lacking and the practice remained a practice nothing more.

Critique of State Practice Opinio Juris-:

  • Both the opinio juris and the state practice requirement suffer from evidentiary problems.
  • With respect to opinio juris, it is extraordinarily difficult to determine whether a state has taken a given action because it believes itself to be bound by a rule of CIL, or if it would have taken the same action in the absence of such a rule.
  • State practice is even more complicated. It is not clear that which practice should be considered as state practice and how much practice is required to qualify as State Practice and Customary International Law.

Whether Acquiseeence (silence) amounts to consent as state practice or not ?

Ans- In Gulf of Maine Case, court defined acquiescence as ‘equivalent to tacit recognition manifested by unilateral conduct which the other party may interpret as consent and as founded upon the principle of good faith and equity.

Generally where states are seen to acquiesce in the behaviour of the other states without protesting against them, the assumption must be that such behaviour is accepted as legitimate.

How much long practice is required to form CIL ?

Ans- It is an important requisite of customary international law but in international law there is no rigid time element and it will depend upon the circumstances of the case and the nature of the usage in question.

  • In North Sea Continental Shelf Case, court held that length of time required to establish CIL vary from subject to subject.

How much uniformity and consistency is required to establish CIL ?

Ans- In Assylum Case, court declared that the customary rule must be in accordance with a constant and uniform usage practised by the states in question. However, in Anglo-Norwegian Fishiries Case, ICJ emphasised its view that some degree of uniformity amongst state practice was essential before the CIL.

Whether Universality or generality is require for CIL ?

Ans- Universality of practice is not necessary, the practice should have generally observed or repeated by numerous state.

Regional custom

  • It is a development of a custom in regional area.
  • Asylum Case - the court declared that the party which relies on a custom of this kind must prove that this custom was established in such a manner that it has become binding on the other party. The standard of proof of such custom is higher than the general custom.
  • For example- India -Sri Lanka fisheries zone.

Local Custom

Local custom depends upon particular activity by one state being accepted by the other state as an expression of a legal obligation or by the other state as an expression of a legal obligation or right.

Instant Customary International Law

  • Cheng’s theory -:, States can advance a new customary international law, either in concert with other states or unilaterally, simply by showing a new opinio juris. If other states do not object, and in fact follow the same, they will share the same opinio juris, thus forming a new rule of customary international law.
  • This becomes important in development of law such Space Law etc.

Advantages of Customary International Law

  1. It is a dynamic process of law creation.
  2. It is Universal and democratic in nature because it involves many people to participate in law making.
  3. CIL mirrors the characteristics of decentralised International legal system.
  4. It is a consensus approach of law making.

Disadvantages of Customary International Law

  1. It is too clumsy and slow moving to accommodate the evolution of International Law.
  2. It is not the best, mechanism to form law in complex situation.


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