Customary international law

Customary international law – or simply ‘custom’ – must be distinguished from the customary law that is an important part of some states’ domestic law and deals largely with family matters, land and suchlike. In international law a rule of custom evolves from the practice of states, and this can take a considerable or a short time. There must be evidence of substantial uniformity of practice by a substantial number of states. In 1974 the ICJ found that a customary rule (now superseded) that states had the right to exclusive fishing within a twelve nautical mile zone had emerged. State practice can be expressed in various ways, such as governmental actions in relation to other states, legislation, diplomatic notes, ministerial and other official statements, government manuals (as on the law of armed conflict), and certain unanimous or consensus resolutions of the UN General Assembly. The first such resolution was probably Resolution 95( ) of 11 December 1946 which affirmed unanimously the principles of international law recognised by the Charter of the Nürnberg International Military Tribunal and its judgment.

When a state that has an interest in the matter is silent, it will generally be regarded as acquiescing in the practice. But if the new practice is not consistent with an established customary rule, and a state is a persistent objector to the new practice, the practice either may not be regarded as evidence of new custom or the persistent objector may be regarded as having established an exception to the new customary rule.

But to amount to a new rule of custom, in addition to practice there must also be a general recognition by states that the practice is settled enough to amount to an obligation binding on states in international law. This is known as opinio juris (not the opinions of jurists). Sometimes the recognition will be reflected in a court judgment reached after legal argument based on the extensive research and writings of international legal scholars. In themselves, neither judicial pronouncements nor favourable mention in a UN resolution, even when adopted by a large majority, are conclusive as to the emergence of new custom.19 But in Nicaragua v. US (Merits) (1986) the International Court of Justice found that the acceptance by states of the Friendly Relations Declaration of the General Assembly constituted opinio juris that the Charter prohibition on the use of force now also represented custom. There is however a growing tendency for international courts and tribunals, without making a rigorous examination of the evidence, to find that a customary rule has emerged.

In Tadic the International Criminal Tribunal for the Former Yugoslavia ruled that it had jurisdiction over war crimes committed during an internal armed conflict even though its Statute does not provide for this.

Establishing opinio juris can be difficult and everything will depend on the circumstances. It is easiest when the purpose of a new multilateral treaty is expressed to be codification of customary international law. Even if the treaty includes elements of progressive development, if it is widely regarded by states as an authoritative statement of the law, and constantly and widely referred to, it will soon come to be accepted as reflecting the customary rules, sometimes even before it has entered into force. This was certainly the case with the Vienna Convention on the Law of Treaties 1969, which even now has only 101 parties. Although many provisions of the UN Convention on the Law of the Sea 1982 (UNCLOS) went beyond mere codification of customary rules in most respects, the negotiations proceeded on the basis of consensus. It was therefore that much easier during the twelve years before UNCLOS entered into force for most of its provisions to become accepted as representing customary law.

An accumulation of bilateral treaties on the same subject, such as investment treaties, may in certain circumstances also be evidence of a customary rule.

Date: 2016-03-03; view: 900