Skip to main content


3.2 International custom accepted as law

State practice

Society and rules are bound together. One could not have games, for example, without rules. Customary international law refers to binding legal rules that have developed on global or region levels through continued practice. The test of the existence of a customary rule of law is the extent to which it is observed in the practice and behaviour of states. Although this may seem a fuzzy or imprecise concept, it provides for flexibility. An example is the emergence and eventual adoption of the exclusive economic zone in the law of the sea. A measure of state practice may be provided by an array of instruments including treaties, declarations, agreements, and other pronouncements. In summary, several factors may be indicative of state practice:

Although no number of states' practice has been identified to establish a custom, the International Court of Justice (ICJ) merely requires such state practice to be a general tendency rather than a minimum number of states' action. It is preferred that the states that participate are widely representative and that the legal rule be consistently followed.

In the Nuclear Advisory Opinion the ICJ referred to the incorporation of Principle 21 of the Stockholm Declaration in the text of many treaties as confirming it as customary law. This is the principle of states having sovereignty over their natural resources and also their responsibility not to cause trans-national environmental harm. As another environmental example, the ICJ in the Gabčíkovo-Nagymaros Case in 1997 approved the principle of 'equitable utilisation' of common water sources when it adjudicated the dispute between Hungary and Slovakia over damming the River Danube (ICJ 1997).

The length of time a rule has been common practice is also relevant in determining its standing. Some rules of maritime law have been accepted for centuries. However, a short period of time does not necessarily disqualify a rule as potentially being customary. In its judgement in the in the North Sea Continental Shelf Cases 1969 (ICJ 1969), the International Court of Justice observed that:

'while a very widespread and representative participation in a convention might show that a conventional rule had become a general rule of international law, in the present case the number of ratifications and accessions so far was hardly sufficient. 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 those 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'.

In a few sentences the ICJ here enunciated the kind of judicial considerations that determine whether or not a rule can be regarded as customary international law.

Apart from the difficulty of determining what is customary international law in practice, there is also the necessity to show that compliance with the practice is based on the view that it is mandated by international law. This need for a legal basis to state practice is termed opinio juris. These requirements mean that the rule must be part of a continuous state practice that should be the result of other states' belief that they are required by law to apply this rule.

Opinio juris

Opinio juris sive necessitates, usually shortened to opinio juris is the belief that a state action or practice is necessarily followed because it is a legal obligation. Evidence of the motive to this state practice can be found in various sources such as:

Customary environmental rules

It is possible to identify obligations in international environmental law which may be considered to be rules of customary international law.

In addition, there are principles of international environmental law that may be increasingly accepted as norms of customary law.