Judgment_paragraph 36 · Criminal proceedings against Ömer Altun and Others (C-359/16)

36In that certificate, the competent institution of the Member State in which an undertaking employing the workers concerned is established declares that its own social security system will remain applicable to those workers. By virtue of the principle that workers must be registered with only one social security system, the certificate thus necessarily implies that the other Member State’s social security system cannot apply (see, to that effect, judgments of 26 January 2006, Herbosch Kiere, C‑2/05, EU:C:2006:69, paragraph 21, and of 27 April 2017, A-Rosa Flussschiff, C‑620/15, EU:C:2017:309, paragraph 38).