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Customary international law

Customary international law is an aspect of international law involving the principle of custom. Along with general principles of law and treaties, custom is considered by the International Court of Justice, jurists, the United Nations, and its member states to be among the primary sources of international law.

Many governments accept in principle the existence of customary international law, although there are differing opinions as to what rules are contained in it. A rule becomes customary international if two requirements are met: (1) There is a state practice that "appears to be sufficiently widespread, representative as well as consistent" showing that a significant number of states have used and relied on the rule in question and the concept has not been rejected by a significant number of states,[1][2] (2) states were motivated by a belief that they were legally compelled to accept the legitimacy of the rule in question because customary international law obligated them to do so (opinio juris).[1]


In 1950, the International Law Commission listed the following sources as forms of evidence of customary international law: treaties, decisions of national and international courts, national legislation, opinions of national legal advisors, diplomatic correspondence, and practice of international organizations.[3][4] In 2018, the Commission adopted Conclusions on Identification of Customary International Law with commentaries.[5] The United Nations General Assembly welcomed the Conclusions and encouraged their widest possible dissemination.[6]

The International Court of Justice[edit]

The Statute of the International Court of Justice acknowledges the existence of customary international law in Article 38(1)(b), incorporated into the United Nations Charter by Article 92: "The Court, whose function is to decide in accordance with international law such as disputes that are submitted to it, shall apply ... international custom, as evidence of a general practice accepted as law."


Article 38(1)(b) of the Statute of the International Court of Justice, has recognized International Custom as evidence of general practice accepted as law. Thus, general practice demonstrates custom, and not vice versa. In order to prove the existence of customary rule, it is necessary to show that there exists a 'general practice' which conforms to the rule and which is accepted as law.


Customary international law "consists of rules of law derived from the consistent conduct of States acting out of the belief that the law required them to act that way".[18] It follows that customary international law can be discerned by a "widespread repetition by States of similar international acts over time (state practice); Acts must occur out of sense of obligation (opinio juris); Acts must be taken by a significant number of States and not be rejected by a significant number of States."[19] A marker of customary international law is consensus among states exhibited both by widespread conduct and a discernible sense of obligation.


The two essential elements of customary international law are state practice and opinio juris, as confirmed by the International Court of Justice in the Legality of the Threat or Use of Nuclear Weapons.[20]


In relation to the psychological element that is opinio juris, the International Court of Justice further held in North Sea Continental Shelf that "not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it ... The States concerned must therefore feel that they are conforming to what amounts to a legal obligation."[21] The Court emphasised the need to prove a "sense of legal duty" as distinct from "acts motivated by considerations of courtesy, convenience or tradition".[21] This was subsequently confirmed in Nicaragua v. United States of America.[22]

Bilateral vs. Multilateral[edit]

The recognition of different customary laws can range from simple bilateral recognition of customary laws to worldwide multilateral recognition. Regional customs can become customary international law in their respective regions, but do not become customary international law for nations outside the region. The existence of bilateral customary law was recognized by the International Court of Justice in the Right of Passage Over Indian Territory case between Portugal and India, in which the court found "no reason why long continued practice between the two states accepted by them as regulating their relations should not form the basis of mutual rights and obligations between the two states".[23]

Other examples[edit]

Other examples accepted or claimed as customary international law include immunity of visiting foreign heads of state and the principle of non-refoulement. In 1993, the United Nations Security Council adopted Geneva conventions as customary international law.


If any treaty or law has been called as customary international law, then parties which have not ratified said treaty will be bound to observe its provisions in good faith.[24]

Customary international humanitarian law

Customary law

Crimes against humanity

Genocide

Human rights

Public international law

International humanitarian law

International human rights law

International regulation

Refugee law

Rule of law

Rule according to higher law

Soft law

Vienna Convention on the Law of Treaties

Customary International Law, Max Planck Encyclopedia of Public International Law

International Committee of the Red Cross

Customary international humanitarian law

with cases and commentary. Nathaniel Burney, 2007.

A Brief Primer on International Law

Customary International Law - Bibliographies on the topics of the International Law Commission (no.13 in the list) (UNOG Library)

Customary IHL Database

Archived 2020-08-27 at the Wayback Machine

Peace Palace Library - Research Guide