Customary International Law in Courts: Developments by the World Court and Other International Courts

Customary International Law on Trial


Customary law has been a source of international law and being so it has been playing its roles in the affairs amongst the international persons (States). By the passage of time and together with the practice and belief of the international persons this category of law has gained a status or norm that administers, legally, the situations with a binding command. The question is- how does it operate? What are the elements that work behind its operation?  Is any derogation from observance permissible? Does it indicate obligations and rights to the international community as a whole? This study will try to examine and or find answers to the questions mentioned above. Finally, this document will leave a space for further discussion and pull a conclusion saying that certain obligations derived from the customary international law are superior to the powers of a sovereign State.


International rights and obligations of international persons are, generally, determined and governed, for example, by international treaties and conventions. In case of any breach one can legally bind others; because rights and obligations are created and documented by them. There are rights and obligations established by state-practice as opposed to obligations arising from formal written international treaties and those may be referred to customary international law [1]. The statute of the ICJ, by Article 38(1)(b) [2], recognized international custom as one of the sources of international law. An effort will be made to find answers to the questions raised above in turn and let me begin with a question of how a customary international law can be established first.

Elements of Customary International law

After closely examining the use of wording used in article 38(1)(b) of the statute of the ICJ it can be found that both the” conduct” and “belief” of a state have been meant for the pre-requisition of a status of a customary international law. One could mark the former one as a physical element and the latter one as a psychological element.  It would be pertinent to mention here the North Sea Continental Shelf cases [3](Germany/Denmark and Germany/ Netherlands) which were decided in 1969 by the ICJ and in this case, the world court set the elements in order to form a binding customary international law. As per this case, one needs to prove two elements, i.e state practice and opinio juris, in order to argue that a customary rule has emerged.

In North Sea Continental Shelf case the parties, interestingly, did not recourse to ICJ to delimit the boundaries of continental shelf but to declare the principles and rules of international law applicable to the delimitation. I would just quote the relevant portion of the judgment that goes with the title of my article:

the two Special Agreements had asked the Court to declare the principles and rules of international law applicable to the delimitation as between the Parties of the areas of the North Sea continenta1 shelf appertaining to each of them beyond the partial boundaries in the immediate vicinity of the coast already determined between the Federal Republic and  Netherlands by an agreement of 1 December 1964 and between the Federal Republic and Denmark by an agreement of 9 June 1965. The Court was not asked actually to delimit the further boundaries involved, the Parties undertaking in their respective Special Agreements to effect such delimitation by agreement in pursuance of the Court’s decision [4].

The Court rejected the contention of Denmark and the Netherlands to the effect that the delimitations in question had to be carried out in accordance with the principle of equidistance as defined in Article 6 of the 1958 Geneva Convention on the Continental Shelf, holding: -that the Federal Republic, which had not ratified the Convention, was not legally bound by the provisions of Article 6; -that the equidistance principle was not a necessary consequence of the general concept of continental shelf rights, and was not a rule of customary international law.

The Court also rejected the contentions of the Federal Republic in so far as these sought acceptance of the principle of an apportionment of the continental shelf into just and equitable shares. It held that each Party had an original right to those areas of the continental shelf which constituted the natural prolongation of its land territory into and under the sea. It was not a question of apportioning or sharing out those areas, but of delimiting them. The Court found that the boundary lines in question were to be drawn by agreement between the Parties and in accordance with equitable principles, and it indicated certain factors to be taken into consideration for that purpose. It was now for the Parties to negotiate on the basis of such principles, as they have agreed to do [5].

………as regards the time element, although the passage of only a short period of time was not necessarily a bar to the formation of a new rule of customary international law on the basis of what was originally a purely conventional rule, it was indispensable that State practice during that period, including that of States whose interests were specially affected, should have been both extensive and virtually uniform in the sense of the provision invoked and should have occurred in such a way as to show a general recognition that a rule of law was involved [6].

In short, two elements, ie state practice and acceptance as law, need to be proved to argue that a customary rule has emerged. And the former is an objective when the latter is a subjective element. The conducts (actions or omissions) of a state must support the custom in order to satisfy the objective element while the state must feel that she is legally bound to perform the custom ( opinio juris) in order to satisfy the subjective element.

Subsequently, in 1986 the ICJ in Nicaragua case [7]clarified that in order for a customary rule to come into force a complete consistency in state-practice is not necessary and the court’s view was reflected in paragraph 186 of the judgment that I quote here below:

 186. It is not to be expected that in the practice of States the application of the rules in question should have been perfect, in the sense that States should have refrained, with complete consistency, from the use of force or from intervention in each other’s interna1 affairs. The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule. In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule. If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State’s conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule.

ICJ in paragraph 207 [8]of the same case explained the opinio  juris in the following words:

207. In considering the instances of the conduct above described, the Court has to emphasize that, as was observed in the North Sea Continental Shelfcases, for a new customary rule to be formed, not only must the acts concerned “amount to a settled practice”, but they must be accompanied by the opinio juris sive necessitatis. Either the States taking such action or other States in a position to 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 a rule of law requiring it. The need for such a belief. i.e., the existence of a subjective element, is implicit in the very notion of the opinio juris sive necessitatis.” (I.C.J. Reports 1969, p. 44, para. 77.)

Non-derogable rights (Jus Cogens)

There are certain rights that have attained so compelling status and norms that derogation from which is not permitted by way of particular agreements. And this is what is called Jus Cogens. There is no exhaustive official list of rights that have attained the peremptory norms, however, the following are commonly accepted being so: the prohibition of the use of force between states, prohibition of genocide, the prohibition of slavery, the prohibition of torture, racial discrimination as well as peoples’ right to self-determination. One would refer article 4(2) of ICCPR [9] to find some rights of peremptory norms that may not be derogated even at the time of public emergency he would, in that respect, cite [10] the following analysis:

“the comments and statements made by the UN Human Rights Committee, a body of independent experts monitoring the implementation of the International Covenant on Civil and Political Rights (ICCPR). In its General Comment No. 29 on States of Emergency, the Committee noted that article 4, para. 2 of the ICCPR prohibits any derogation from article 6 (right to life), article 7 (prohibition of torture or cruel, inhuman or degrading punishment, or of medical or scientific experimentation without consent), article 8, paras. 1 and 2 (prohibition of slavery, slave-trade and servitude), article 11 (prohibition of imprisonment because of inability to fulfil a contractual obligation), article 15 (the principle of legality in the field of criminal law – is crimen, nulla poena sine lege), article 16 (the recognition of everyone as a person before the law), and article 18 (freedom of thought, conscience and religion). The same applies, in relation to States that are parties to the Second Optional Protocol to the Covenant, aiming at the abolition of the death penalty, as prescribed in article 6 of that Protocol. One of the conditions for the justifiability of any derogation from the Covenant is that the measures taken do not involve discrimination solely on the ground of race, colour, sex, language, religion or social origin. Even though article 26 or the other Covenant provisions related to non-discrimination (articles 2, 3, 14.1, 23.4, 24.1, and 25) have not been listed among the nonderogable provisions in article 4, para. 2, there are elements or dimensions of the right to non-discrimination that cannot be derogated from in any circumstances. In particular, this provision of article 4, para. 1, must be complied with if any distinctions between persons are made when resorting to measures that derogate from the Covenant.  The Human Rights Committee believes that «The fact that some of the provisions of the Covenant have been listed in article 4 (paragraph 2), as not being subject to derogation does not mean that other articles in the Covenant may be subjected to derogations at will, even where a threat to the life of the nation exists». The relevant article 4 requires that no measure derogating from the provisions of the ICCPR may be «inconsistent with the State party’s other obligations under international law, particularly the rules of international humanitarian law».”

Pointing at the state-practice combined with the opinio juris one can pull a conclusion, on the statements made above by the Committee, that the nonderogable rights quoted in article 4 of the ICCPR were considered by its drafters a part of international customary law or he can go further pulling that those rights, at least, have reached that status after the ICCPR entered into force in 1979.

Erga Omnes Rights and Obligations

The concept Erga Omnes [11] has been used in international law as a legal term describing obligations owed by states towards the community of states as a whole. The nature of this concept is such that if a state is of a breach of the said rule any state can come forward with a complaint of that breach by the other state; because every state has an interest in the protection of the rules that generate erga omnes obligations. The concepts of jus cogens and erga omnes are closely related and it is difficult to draw a difference between them. The former creates the latter erga omnes obligations for the states to comply and if this analysis is right it necessary follows that an erga omnes obligation is the consequence of a rule being characterized as jus cogens.

According to the 1969 and 1986 Vienna Conventions on the Law of Treaties, a treaty is void if it conflicts with jus cogens (Art. 53 [12] and 64 [13]). I would like to mention here some cases where ICJ and other international courts decided erga omnes obligations. For example, in Barcelona Traction case [14] the ICJ enumerated four erga omnes obligations; the outlawing of acts of aggression, the outlawing of genocide, protection from slavery, and protection from racial discrimination. Subsequently, obligations to respect the principles of self-determination and obligations prohibiting the use of torture were recognized as erga omnes obligations by the courts in East Timor [15] and Furundzija [16]case respectively.


Let us go for few instances before pulling off a conclusion:

X is a sovereign state and has already ratified those conventions that prohibited acts concerning rights mentioned in preceding paragraph. Can she legislate anything for her own nationals derogating from the ratifying instruments- because she is a sovereign state?

Would the answer differ in a position where the state be a new one and did not ratify any of the conventions guaranteeing rights mentioned above and even is not a member of the United Nations? Pointing at the concepts of jus cogens and erga omnes obligations one would argue that erga omnes obligations are superior to the powers of a sovereign state and he would answer to the questions posed above with a simple word; and that is “NO”.





[4] ibid

[5] ibid

[6] ibid


[8] ibid


[10] ISPI, WP-7, Customary law as an Instrument for the Protection of Human Rights, Vojin Dimitrijevic., accessed on 30.8.2018.


[12] Vienna Convention on the law of treaties (with annex). Concluded at Vienna on 23 May 1969,, accessed on 30.8.2018.

[13] Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations 1986,, accessed on 30.8.2018.



[16] ICTY, Prosecutor v Furundzija, 1998, para 151.

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Muhammad Muzahidul Islam

Muhammad Muzahidul Islam

Muhammad Muzahidul Islam studied LL.B (Hons) from the University of London, Bar Vocational Course from the City University London, LL.B(Hons),LL.M from Rajshahi University, Bangladesh, Barrister-at-Law (Lincoln’s Inn, London). He is an advocate of the Supreme Court of Bangladesh and a human rights activist.
Muhammad Muzahidul Islam

Muhammad Muzahidul Islam

Muhammad Muzahidul Islam studied LL.B (Hons) from the University of London, Bar Vocational Course from the City University London, LL.B(Hons),LL.M from Rajshahi University, Bangladesh, Barrister-at-Law (Lincoln’s Inn, London). He is an advocate of the Supreme Court of Bangladesh and a human rights activist.

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