Abstract: Coating cardboard, for producing packaging, formed of a (natural-) fibre-based support layer covered on at least one of its faces by a pigment layer, includes a fluorescent compound disposed discontinuously on the surface of the support layer, between the support layer and the pigment layer, the compound being invisible to natural light and visible under UV irradiation through the pigment layer, and constituting a marking element of the cardboard, without altering the printability properties of the outer face of the pigment layer. In the method of marking a coated cardboard, the marking element is applied according to a discontinuous surface, by an aqueous solution or dispersion containing the fluorescent compound, to the surface of the face of the support layer which is intended to be covered subsequently by the pigment layer, by for example flexographic printing or spraying. The marking element may serve as a distinctive sign or security sign.