May 26, 2024
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This article has been written by Gopika Kalidas Naduvilath, a fifth-year law student pursuing BBA LLB (Hons) at Alliance University, Bangalore. 


What is the origin and main source of international law? It is very crucial to recognize the sources of international law, as there is no strict ‘Code of International Law.’ It has been derived from different sources. The sources of international law created the framework for international law practice, especially public international law. The main authority that has identified the sources of international law is the Statute of the International Court of Justice[1] (ICJ Statute).  The sources of international law have been defined under Article 38 of the ICJ Statute. The sources under Article 38 are as follows: –

  • International conventions
  • International customs or customary international law
  • The general principles of law
  • Judicial decisions
  • Juristic works

International Customs

Also known as customary international law, is given under Article 38(1)(a). The oldest source of law is known as ‘Custom’. They are original laws that have historically been followed and have gained recognition around the world. A major part of it has been derived from state practices or society’s rules. The creation of customs is slow and uncertain, along with being replaced by other sources, but still, custom is the base and contributes significantly to international law. Customary international law is not written but has strong traces of being followed by the community.

To further understand the meaning of ‘custom’, it is also necessary to know the meaning of the word ‘usage’. The ‘usage’ is the actions that are often repeated by states, whereas custom arises when a clear and persistent habit of doing certain acts has been established on grounds of belief in them to be right. This leads to custom, is a usage of law and is treated as such because it has not received the force of law. The true test of recognizing a custom is the general consensus of opinions along with the continued usage of those rules, which has become an accepted practice in various states. This leads us to understand the two factors considered when it comes to proving a law is customary, which are: –

  1. There must be objective evidence of state practice and 
  2. the international community must believe that such practice is required as a matter of law – this subjective element is known as opinio juris. Once there is sufficient practice together then, a new custom emerges, but mostly these new practices are rejected at the starting stages themselves to avoid any kind of application of the same.

The criteria for the two elements required to identify custom were recognized in North Sea Continental Shelf case 1969 (Federal Republic of Germany (FRG) V Denmark; FRG v The Netherlands). Further, the case of SS Lotus 1927 (France v Turkey) also laid the foundation for understanding custom as a source. Customary law, unlike treaties, is binding upon all the states, regardless of whether they are a signatories Customary international law is limited as it is not codified, which makes it unclear and less specific than a written treaty except for the practice of these rules. Customary international law as a source is of fundamental importance as it acts as the basis for deriving international law.

International Conventions

Also commonly known as treaties given under Article 38(1)(a) plays a very important role in international law. These are written agreements and could be multinational or bilateral treaties. These agreements are signed by various states and ratified making an obligation on them to follow those laws. The purpose of international treaties is to enhance international cooperation. One of the advantages of international agreements compared to customary international law is that they are written, which makes it easier for them to be proven as opposed to unwritten international law.[2] The Vienne Convention on the Law of Treaty (VCLT) 1969, [3]defines ‘Treaty’ as ‘‘an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation’ (Article 2(1)(a)).”

Treaty law is based on the principle of ‘pacta sunt servanda’, which means that agreements have to be respected and complied with. In accordance with Article 18 of the VCLT, Member States are required to refrain from taking any measures that would hinder the conclusion of this Agreement. This is under the condition that it has signed the treaty that has been subsequently ratified. It does not wish to be part of the treaty until then. This is also dependent on the fact that it did not have an undue delay in joining the treaty. In practice, the adoption of a number of treaties in different areas has been done in international law that, till date, has also gone through significant changes. A few of the significant international treaties are as follows: –

  • United Nations Charter 1945.
  • ICJ Statute 1945.
  • The Geneva Conventions 1949.
  • Universal Declaration of Human Rights (UDHR) 1948.
  • International Covenant on Civil and Political Rights (ICCPR) 1976.
  • The United Nations Convention on the Law of the Sea (UNCLOS) 1982.

The General Principles of Law

The general principles of law are those principles of law in general, which are not limited to international law but other aspects of law like civil, criminal, administrative, environmental, and others. As Seen under Article 38(1)(c) the general principle of law is mainly derived from the domestic laws of states and is accepted for international order. Some of the general principles of law are: –

  • good faith;
  • res judicata;
  • prescription;
  • indemnity;
  • subrogation;
  • estoppel and other principles.

General principles of law, which have been accepted by civilized countries as a third source, are rarely mentioned in judgments. They are widely used where the ICJ or other international tribunals would like to introduce concepts such as the legal personality of a company, for instance in Barcelona Traction Company case of 1970 (Belgium v Spain) and the Chorzow Factory case of 1928 (Germany v Poland), which have been widely accepted in national legal systems and the international community.

Judicial Decisions

Judicial decisions are referred to under Article 38(1)(d) are the various decisions given by international and domestic courts that are one of the means of recognizing the sources of international law. But in practice, the ICJ cannot refer to domestic decisions, but it does contribute directly to the formation of customary law. This is especially because of the rule of ‘stare decisis’ where the decision of the international court has no binding force except on the parties involved in that particular case. This is also given under Article 59 of the ICJ Statute.

Juristic Works

The justice teachings and works, referred to under Article 38(1)(d), from prominent scholars, judges, legal personalities, etc. are also a major contribution to the source of international law. Even though it has no legal binding, it’s contribution to the development as a source of international law is present. Alberico Gentili, Alfred Pedro, Hugo Grotius and Emmerich de Vattel are a few of the remarkable people known for their contribution to international law through their works and theories. Calvo’s doctrine and Hull’s theory are a few other examples.

Currently, when we look at the modern source of international law, it is primarily the resolutions made by the U.N.’s principle organ, the General Assembly, and their established body, the International Law Organization, which has emerged as one of the most important components of international law.


The sources of international law are diverse and comprehensive in view of the complexity of global governance and international relations. Treaties, common law, general principles, judicial decisions, and academic papers are the means by which international legal standards are developed, interpreted, and enforced. These resources need to be understood so that the complexity of international law can be managed and cooperation and accountability within the community can be promoted. By recognizing and respecting these sources, states can promote the rule of law, support peace, justice, and prosperity on a global level.

[1] Statute of the International Court of Justice,

[2] Anthony Aust, Handbook of International Law, Cambridge University Press, 2010.

[3] The Vienne Convention on the Law of Treaty 1969,

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