26However, according to the Court’s settled case-law, the principle of equal treatment, which is a concept of European Union law, prohibits not only overt discrimination, based on nationality, but also all covert forms of discrimination which, through application of other criteria of differentiation, lead in fact to the same result (see, by way of example, Case C-278/94Commission v Belgium [1996] ECR I-4307, paragraph 27 and the case-law cited). That case-law, which was already in existence at the date of the signature of the Agreement, is also valid with regard to the application of that Agreement, in accordance with Article 16(2) thereof.