33It is clear from settled case-law that, given the fundamental importance of the principle of equal treatment, the exception to the prohibition of discrimination on grounds of sex, provided for in Article 7(1)(a) of Directive 79/7, must be interpreted strictly (see, in particular, Case 152/84 Marshall [1986] ECR 723, paragraph 36, and Case C-328/91 Thomas and Others [1993] ECR I-1247, paragraph 8). That provision can apply only to the determination of pensionable age for the purposes of granting old-age and retirement pensions and to the consequences thereof for other social security benefits (Case 151/84 Roberts [1986] ECR 703, paragraph 35; also to that effect, Case C-303/02 Haackert [2004] ECR I-2195, paragraph 30). That exception to the prohibition of discrimination on grounds of sex is therefore not applicable in the case of a tax concession such as that at issue in the main proceedings, which is not a social security benefit.
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