Customary International Law as a Source of Law

Note: This blog post maybe outdated. 

An Introduction to Sources of Law

Article 38 (1) of the Statute of the International Court of Justice (ICJ) is considered as an authoritative statement on sources of international law.

The ICJ is requested to form its opinion based on customary international law evidenced by “general practice accepted as law”. This sets out the two essential components necessary to form customary international law, which the Court interpreted as State practice and opinio juris.

In the North Sea Continental Shelf Cases, the ICJ held that in order to argue that a customary rule has emerged one needed to prove:

(1) The objective element (State practice).

In other words:

(a) widespread and representative participation, including States whose interests were specially affected (i.e. generality); and

(b) virtually uniform practice (i.e. consistent and uniform usage) undertaken in a manner that demonstrates;

(2) a general recognition of the rule of law or legal obligation (i.e. opinio juris).

In the North Sea Continental Shelf Cases the ICJ held that the passage of a considerable period of time was unnecessary (i.e. duration) to form a customary law.

In summary, two elements are necessary before one could argue that a customary rule has emerged:

1. The Objective Element: The existence of State practice. In other words, the actions or omissions by the State must support the custom; and

2. The Subjective Element: Acceptance as law. In other words, States when performing a custom must do so because they feel that they are legally bound to perform the custom. We call this opinio juris.

Figure 1: Elements of Customary International Law

Elements of Customary International Law
Elements of CIL

State Practice

State practice can be seen in the actual actions performed by the State (both by way of acts and omissions), statements made by authorized representatives in international fora, or through national laws and judicial decisions that deal with international relations. ✐  See further the ILA report on the formation of CIL paras. 7-11.

“State practice” that is necessary for the formation of a CIL  must be (1) consistent and uniform; (2) generally accepted by States; and (3) of a certain duration. The latter, as we saw in the North Sea Continental Shelf Cases, does not need to be excessively long.

On State practice, the jurisprudence of the Nicaragua case contained several important clarifications in respect of inconsistent State practice (para 186). The ICJ held that:

(1) For a customary rule to come into force, it is not necessary to have complete consistency in State practice in respect of the rule.

(2) Inconsistent State practice does not affect the formation or existence of a customary principle so long as the inconsistency is justified by the State as a breach of the rule.

(3) This attempt at justifying a violation would only make the rule’s customary law nature stronger.

Opinio Juris

Click here for the blog post on opinio juris.

Regional Customs

Customary international law can be general or particular. Particular customary law embodies local or regional customs. This type doesn’t bind all States; but, binds some States that share a common interest or that are in a similar geographical location. See the Asylum Case (Summary) where the ICJ had to decide on the existence of a regional custom.

© Ruwanthika Gunaratne and Public International Law at, 2008 – 2020. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Ruwanthika Gunaratne and Public International Law with appropriate and specific direction to the original content.

12 thoughts on “Customary International Law as a Source of Law

  1. dear miss,

    This website has been a great resource for me to understand public international law more better.

    concerning customary international law and treaty law , i dont see anything regarding the case of para military activity in and against nicaragua vs united states.

    isn’t this case an important point of the development in public international law and the ICJ?

    Kind regards,
    student international & european law
    @ the hague university

    1. Hi Noah,

      Many thanks for your message. This is a very important case: both in terms of CIL and Treaties (inter-relationship between the two) and on the use of force. It is also important for the ICJ because the ICJ continued to hear the case against the United States – despite its objections on jurisdiction – and found againt the United States. There is a reference to the Nicaragua case in lesson 5.4. on the definition of an armed attack. There is an entire AJIL edition (?) dedicated to the Nicaragua case, as I remember. Let me find that and send you the reference.


  2. case is indeed very useful in environmental law when one discusses about the principles of international law and international customary law as one of the sources of international environmental law.

  3. Hello my name is Francis Banda, i have been overwhelmed by your comments and answers to questions to do with international law. Please keep it up and hope one day i can emulate. Am a 3rd year student of Law here in Zambia. and your research has been of help to me.

  4. In the Nicaragua v. United States case(1984) ICJ Rep. 392, the ICJ opined as follows: ” Not only must the acts concerned amount to a settled practice, but they must also be accompanied by the opinio juris, either the state taking such actions or other state in a position react to it must have behaved so that their conduct is evidence of a belief that this practice is rendered obligatory by the existence of the rule of law requiring it.” With reference to relevant principles of international law, give your understanding of the above statement with the aid of authorities where necessary. PLEASE COULD HELP WITH THIS QUESTION I SEEM TO BE STUCK

  5. Thank you very much for sharing your expert knowledge with the rest of the world. Your efforts are much valued and appreciated.

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