The labels used in a labelling machine for containers, in particular bottles, are separated by a roll (2), which is mounted on a central pin (4), and is locked against a support surface (P) by a locking ring (7) provided with a sleeve (8) which is coupled to the central pin (4) in a sliding manner, and with at least two coupling blocks (16) which are mobile, relative to the sleeve (8), between an extracted operating position, wherein the coupling blocks (16) are fastened to the central pin (4), and a retracted rest position, wherein the coupling blocks (16) disengage the central pin (4) itself.