The United Kingdom’s decision to leave the European Union, commonly referred to as Brexit, was the first of many unexpected developments in British politics over the course of the last three years. In the last few days, international lawyers have been surprised by the attempt to invoke article 62 of the Vienna Convention of the Law of Treaties in order to facilitate the passing of a third vote by the UK Parliament of the Withdrawal Agreement.

Several commentators have explained the flaws inherent to the application of a provision requiring a fundamental change of circumstances that cannot be foreseen (see the excellent blog by Marko Milanovik here, including comments by Alessandra Asteriti & Aoife O’Donogue),  and for which an extraordinarily high threshold was established by the International Court of Justice in the Gabčíkovo-Nagymaros Project case (commentary by Philippe Sands who represented Hungary before the Court here). In successive tweets Matin Paparinskis has unearthed the historic opposition of the UK to the doctrine of fundamental change of circumstances and suggested that arguments based on the doctrine of necessity may provide better legal grounds for the unilateral termination of a treaty.

The applicability of article 62 underlines every analysis I have encountered so far. Commentators refer to the Vienna Convention of the Law of Treaties without specifying whether they are referring to the 1969 Convention only applicable to treaties between states, or the 1986 Convention for treaties between States and International Organisations (the latter not yet into force). This is not an oversight. The clarification seems unnecessary because article 62 is presumed as a codified expression of customary international law that applies to any treaty, whether concluded between States or between States and international organisations.

The International Court of Justice has supported this view on several occasions. In its admissibility decision on the Qatar v Bahrain case, the Court applied the definition of treaty enshrined in article 2 of the 1969 Vienna Convention of the Law of Treaties, despite neither Qatar nor Bahrain being parties to the Convention. The Court did not justify its approach implicitly suggesting that the definition contained in the Vienna Convention had customary law status, a conclusion categorised as ‘surprising’ by Jan Klabbers ‘given the fact that it is, after all,  but a definition, and, moreover, a definition for purposes of the Vienna Convention only’ (page 366). The Court was forced into a more explicit approach in Gabčíkovo-Nagymaros because Hungary argued that the Vienna Convention was not applicable to the concerned agreement, suggesting that the Court should establish ‘the conformity of the prescriptions of the Convention with customary international law’ on a case-by-case basis (paragraph 43). The International Court resolved the issue in a sweeping statement only substantiated by its own jurisprudence (paragraph 46):

“The Court has no need to dwell upon the question of the applicability in the present case of the Vienna Convention of 1969 on the Law of Treaties. It needs only to be mindful of the fact that it has several times had occasion to hold that some of the rules laid down in that Convention might be considered as a codification of existing customary law. The Court takes the view that in many respects this applies to the provisions of the Vienna Convention concerning the termination and the suspension of the operation of treaties, set forth in Articles 60 to 62 (see Legal Consequences for States of the Continued Presence of South Africa in Namibia (South West Africa) notwithstanding Security Council Resolution 276 (1970), Advisory Opinion, I.C.J. Reports. 1971, p. 47, and Fisheries Jurisdiction (United Kingdom v. Iceland), Jurisdiction of the Court, Judgment, I.C.J. Reports 1973, p. 18; see also Interpretation of the Agreement of 25 March 1951 between the WHO and Egypt, Advisory Opinion, I.C.J. Reports 1980, pp. 95-96).”

The International Court of Justice has repeatedly been unwilling to support its statements referring to the existence of customary law, using state practice and/or evidence of opinio juris (for a recent criticism referring to the Chagos case, see Milanovic here). International lawyers should be careful in adopting the same method when it is difficult or impossible to corroborate the existence of a customary rule. Reference to the jurisprudence of the Court does not suffice because the Court has failed to engage, for the most part, with proper analysis in this area. The fundamental change of circumstance’s clause, where acceptance of the validity of the rule can be derived from general consensus of opinion among states rather than state practice, might fit better the categorisation of general principle of international law.