Identifying Customary International Law: From the International Law Association’s Principles of 2000 to the International Law Commission’s 2016 draft

Identifying Customary International Law, Event Summary

On 19 October 2016, Professor Maurice Mendelson QC, the former Chair of Public International Law at UCL and former Rapporteur and Chairman of the International Law Association (ILA) Committee on Formation of Customary (General) International Law spoke at UCL. Professor Mendelson discussed the work of the ILA and International Law Commission (ILC) on the formation and identification of customary international law. At the outset he stressed that, the task of setting out the principles on the formation of customary law was not an easy one, but very much needed. This is because, apart from the academic interest in the elucidation of the issue, it possesses the potential to assist domestic courts and practitioners in municipal jurisdictions when faced with questions relating to customary international law, as well as to those who wished to study international law.

The speaker drew comparisons between the Statement of Principles Applicable to the Formation of General Customary International Law adopted by the ILA in 2000 and ILC’s Report on Identification of Customary International Law issued earlier this year.

Professor Mendelson observed that the ILC’s Report was closely modelled on the ILA’s work, thus there are indeed great many similarities between the two. Both institutions drew largely on the same sources, mainly but not exclusively ICJ decisions, and adopted the same technique of stating a “principle” (ILA) or a “conclusion” (ILC) accompanied by a commentary. He noted that, whilst the ILA reports spoke of “formation” of customary law, the ILC spoke of “identification”. The title of the ILA committee had not been his choice, and overall he preferred the ILC’s term; however, it was to be appreciated that this did not, in many cases, enable the issue of an alleged rule’s pedigree to be sidestepped.

There was no substantial disagreement between the ILC and ILA with regard to issue of state practice as an element of a customary international law rule, although slightly different terms were used to describe any such state practice. According to the ILA, for instance, it had to be ‘virtually uniform’ (Principle 13). The ILC, conversely, referred to ‘sufficiently widespread and representative, as well as consistent’ practice (Draft conclusion 8).

According to Professor Mendelson, the reports take similar approaches to the ‘persistent objector’. The ILC report contained a short section on particular customary law (including but not limited to local and regional); the remit of the ILA Committee had precluded examination of this topic, but he had gone into it in some detail in his lectures to the Hague Academy of International Law, and he gave some examples of particular customary law where the criterion was not geographical.

Where the ILA and ILC palpably disagreed was the role of the opinio juris, what is usually said to be the second element of a rule of customary international law. The ILC Report espoused the more orthodox view that both state practice and opinion juris were necessary conditions for a rule to be considered as customary international law. Professor Mendelson believed that, although opinio juris had a role to play in certain circumstances, it was not always necessary to prove its existence in order to show that a rule amounts to customary international law. Presence of widespread and representative state practice could be sufficient indication of existence of customary international law. He also rejected the argument that opinion juris was necessary in order to distinguish customary international law from comity. It was usually the context in which the state practice occurred that allowed to make the distinction. Finally, the ILC’s use of term ‘accepted as law’ so as to indicate the subjective or psychological element, superficially circumvents some of the problematic aspects of opinion juris. ‘Accepted as law’ does not necessarily mean that states believed something to be law. It could also mean that states wanted it to be law or accepted it without holding a belief that it was an existing rule of law.

The ILC and ILA concurred on the point that resolutions of the UN General Assembly as a general rule and of itself did not create a rule of customary international law (Principle 28 of the ILA Statement of Principles and Draft conclusion 12 of the ILC Report). However, in its Principle 32, the ILA proposed an exception: ‘Resolutions accepted unanimously or almost unanimously, and which evince a clear intention on the part of their supporters to lay down a rule of international law, are capable, very exceptionally, of creating general customary law by the mere fact of their adoption.’ Professor Mendelson emphasised that this reflects very exceptional and extreme circumstances in which all the states had to be united in the face of a common challenge.

Finally, Professor Mendelson concluded that it could hardly be expected that the product of the ILC’s work on customary international law could become a treaty but it may gain clout as an important source of guidance and reference on the issue.