wherein the retaining ring comprises: an engagement element which projects inwards from the free edge and is configured to engage with a locking ring of the neck, the engagement element being in a bent configuration when the cap is in the closed condition; a first incision line and a second incision line, which are transversal to the axis and are positioned circumferentially at the sides of a connecting portion, which is positioned at least in the joining zone and extends for a second angle ; the first incision line and the second incision line being configured to allow deformability at least of the connecting portion and of a region of the engagement element, https://prnt.sc/5Djr9_dOYHqw