Sources of international law

1. Article 38(1) of the ICJ Statute

The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:

• (a) international conventions, whether general or particular, establishing rules expressly recognised by the contesting States;
• (b) international custom, as evidence of a general practice accepted as law;
• (c) the general principles of law recognised by civilised nations;
• (d) subject to Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.

Article 38(2) adds that this does not prejudice the power of the Court to decide a case ex aequo et bono if the parties agree. That power has never been used.

Two points of structure. First, (a) to (c) are formal sources; (d) is only a subsidiary means of finding the law. Second, there is no strict hierarchy between treaty and custom, but in practice a treaty between the parties is applied first as the more specific rule, unless the custom is jus cogens.

2. Treaties

A treaty is an international agreement concluded between States in written form and governed by international law (VCLT 1969, Article 2(1)(a)). Treaties bind the parties under the rule pacta sunt servanda (Article 26). Writers divide them into:

• Law-making treaties (traités-lois), multilateral treaties laying down general rules for many States: the UN Charter, the Genocide Convention 1948, the Vienna Conventions of 1961 and 1969, UNCLOS 1982. These are the closest thing to legislation in international law.
• Treaty-contracts (traités-contrats), bilateral bargains such as a trade or extradition treaty. They bind only the parties, but a series of similar treaties can produce a customary rule, as with extradition and consular rules.

A treaty does not create obligations for a third State without its consent (Article 34). It can, however, reflect or crystallise a custom which then binds everyone (Article 38 of the VCLT).

3. Custom

Custom is the oldest source and still the source of most general rules. Article 38(1)(b) describes it as evidence of a general practice accepted as law. It has two elements.

• State practice (the objective element): what States actually do and say, in diplomatic correspondence, legislation, decisions of national courts, official statements, votes and military manuals. The practice must be constant and uniform, and general, though not universal.
• Opinio juris sive necessitatis (the subjective element): the practice must be followed out of a sense of legal obligation, not habit, courtesy or convenience. Practice without it is mere usage (for example, ceremonial salutes at sea).

The leading cases.

• S.S. Lotus (France v. Turkey), PCIJ, 1927. France argued that only the flag State could prosecute for a collision on the high seas, pointing to the absence of prosecutions by other States. The Court held that abstention was not enough: it would show a custom only if the abstention were based on a conscious duty to abstain. This is the classic statement of opinio juris.
• Asylum Case (Colombia v. Peru), ICJ, 1950. Colombia relied on a regional Latin-American custom allowing the State granting diplomatic asylum to qualify the offence unilaterally. The Court held that the party relying on a custom must prove it is in accordance with a constant and uniform usage; the practice was too uncertain and contradictory. The case also confirms that regional or local custom can exist.
• North Sea Continental Shelf (Federal Republic of Germany v. Denmark and Netherlands), ICJ, 1969. The equidistance principle in Article 6 of the 1958 Geneva Convention on the Continental Shelf did not bind West Germany, a non-party, as custom. For a treaty rule to pass into custom there must be very widespread and representative participation, including States whose interests are specially affected; practice that is extensive and virtually uniform; and opinio juris. A short period of time is not itself a bar.
• Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. USA), ICJ, 1986. The Court applied customary law (because of a US reservation excluding multilateral treaties) and held that the prohibition of force and the principle of non-intervention exist as custom alongside the Charter. Practice need not be perfect: conduct inconsistent with a rule, if treated as a breach rather than as a new rule, confirms the rule.
• Anglo-Norwegian Fisheries (UK v. Norway), ICJ, 1951. A State that has consistently objected to a custom while it is forming is not bound by it: the persistent objector rule. Norway had always opposed the ten-mile rule for bays.

4. General principles of law

These are principles common to the major legal systems of the world, used to fill gaps where treaty and custom are silent and so avoid a non liquet. Examples applied by international tribunals:

• reparation for a breach of an obligation (Chorzów Factory, PCIJ, 1927 and 1928);
• estoppel and acquiescence (Temple of Preah Vihear, ICJ, 1962);
• res judicata, good faith, and that no one may be judge in his own cause;
• circumstantial evidence, accepted in Corfu Channel (UK v. Albania), ICJ, 1949.

The phrase “civilised nations” is now read to mean all nations; the colonial overtone is not applied.

5. Judicial decisions and writers

Article 59 of the Statute provides that a decision of the Court has no binding force except between the parties and in respect of that particular case. There is no formal stare decisis. In practice, the Court follows its own earlier decisions closely, and its statements in Reparation, Lotus and North Sea are treated as settled law. Decisions of arbitral tribunals (Island of Palmas, 1928; Trail Smelter, 1938 and 1941) and of national courts (The Paquete Habana, US Supreme Court, 1900) are also used.

Writers were once the main source (Grotius, Vattel). Today their work is evidence of the law. The reports and draft articles of the International Law Commission, such as the 2001 Articles on State Responsibility, carry special weight.

6. Sources outside Article 38

• General Assembly resolutions. Under the Charter they are recommendations, not binding (except on internal matters such as the budget). But they can be evidence of opinio juris: the Court so treated the Friendly Relations Declaration (Resolution 2625, 1970) in Nicaragua, and Resolution 1514 (1960) on decolonisation in the Chagos advisory opinion, 2019.
• Unilateral declarations made publicly and with intent to be bound (Nuclear Tests, Australia and New Zealand v. France, ICJ, 1974; the Ihlen declaration in Eastern Greenland, PCIJ, 1933).
• Decisions of international organisations, such as Security Council decisions under Chapter VII, which bind members by Article 25 of the Charter.
• Equity, used within the law, as in the equitable delimitation of the continental shelf in North Sea.

7. Jus cogens

Article 53 of the VCLT defines a peremptory norm as a norm accepted and recognised by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a later norm of the same character. A treaty that conflicts with such a norm is void; Article 64 adds that a treaty conflicting with a newly emerged peremptory norm becomes void and terminates. Accepted examples: the prohibition of aggressive force, genocide, slavery, torture, racial discrimination and the right of self-determination. Jus cogens is a limit on the consent theory: it binds whatever the State has agreed.

How this is examined

February 2025 set “What are the sources of international law?” as the essay; June 2025 asked for the sources with an explanation of treaties as a source; August 2024 set a short note on customs. The structure is: Article 38(1) quoted, treaty (law-making and treaty-contract, Articles 26 and 34 VCLT), custom with both elements and the four cases, general principles with examples, Article 59 and the subsidiary means, then a paragraph on resolutions and jus cogens. Quoting Article 38 accurately earns easy marks.