77It is generally recognized that the First Vienna Convention codifies certain universally binding rules of customary international law and that hence the Community is bound by the rules codified by the Convention. This is confirmed by the fact that on several occasions the Court of Justice has relied upon provisions of the Convention when interpreting international agreements concluded by the Community, including the FTA (see, for instance, Opinion 1/91, cited above, paragraph 14, and the judgment in Case C-312/91Metalsa [1993] ECR I-3751, paragraph 12).