Customary Law - International Law

For a rule to be seen as a rule of customary IL which binds all states, there are two requirements: consistent and general state practice and opinio juris (North Sea Continental Shelf Cases).

• There may be regional customs in particular areas (Asylum case)

• New custom will override old customary law

‘State practice’ is practice showing that states are acting in compliance with a particular rule. There must be consistent and general (not erratic) state practice by a significant majority of states.

1. State practice need not be absolutely uniform and universal , as long as it is extensive and ‘virtually uniform’(North Sea Continental Shelf Cases);

2. State practice must be, in general, consistent with the rule and instances of inconsistent conduct must have been treated as breaches of the rule rather than recognition of a new rule (Nicaragua Case)

3. State practice need not have been occurring over a period of extensive duration, as long as these requirements are shown (North Sea Continental Shelf)

   • Custom can arise almost instantaneously between two states if there are no other claims (eg rules applicable to the use of outer space between the USA and USSR)

4. The states ‘most affected’ - those with relevant interests at risk - must have acceded in the purported rule (North Sea Continental Shelf Cases/Legality of Nuclear Weapons Case)

Evidence of state practice is ‘any act or statement by a state from which views about customary law can be inferred’ (Akehurst, quoted in Trigg). Evidence includes:

‘Opinio juris’ is a belief that the practice the state is undertaking is a legal obligation and that it is bound to follow the rule. This subjective element must be proved by evidence that the rule ‘is of such a nature, and has been so widely and generally accepted, that it can hardly be supposed that any… state would repudiate it’ (West Rand Central Gold Ming Co).

• Given the difficulties of evidencing this subjective requirement, judicial inquiry has sometimes return to examination of state practice (see, eg, Congo v Belgium, where the majority of the ICJ confined its analysis to state practice); however, opinio juris is often nonetheless held to be important (Legality of Nuclear Weapons)

Evidence of opinio juris

• GA (see, eg, Israeli Wall case, Nicaragua, Kosovo Advisory Opinion) and SC resolutions (see, eg, Israeli Wall case): these are often considered by tribunals without pointing to whether they reflect state practice or opinio juris, showing they are evidence of both

• Treaties are not evidence of state practice, since a state obeying a rule because it is...