The emergence of European citizenship in 1992 was considered an important step forward in the “constitutionalization” of the Community legal order. As it was formulated, European citizenship was little more than a compilation of the rights contained in the founding Treaties and secondary law. But the CJEU, supported by a prointegrationist academic doctrine, turned it into the “fundamental status of the nationals of the Member States”. It is not surprising then that many scholars considered Brexit, which involved the loss of European citizenship for millions of UK nationals, a disappointment. Some of them looked at international law trying to find some limits to the most serious effects of Brexit on the rights of European citizens. The aim of this article is precisely to analyse in detail those doctrinal discourses that resort to international law as a possible constraint on state sovereignty. At the end we will see that these proposals are based not only on a methodological misperception of what international law is and what it is for, but also on a serious distortion of the real meaning of European citizenship.