Article 53 Vienna Convention on the Law of Treaties

Treaties conflicting with a peremptory norm of general international law (“jus cogens”)

A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law. For the purposes of the present Convention, a peremptory norm of general international law is a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character.

See also: Article 52 Vienna Convention on the Law of Treaties