Abstract: The screw cap has a guarantee ring (3) connected to the cap (2) by means of thin flanges (4). It has its maximum thickness (d) in the vicinity of said flanges and the thickness decreases upwards. The height (h) of ring (3) is at the maximum half that of screw cap (2). Thus, the ring is strong enough not to be pressed in radially, while only requiring the absolute minimum of material necessary for its manufacture. It covers a milled surface (6) on the screw cap (2) which, after removing the same, serves for screwing down the cap. The outer surface of ring (3) and the uncovered part (2d) of screw cap (2) have a smooth surface. This makes it impossible to open the cap for the illegal removal of the content of container (1) without breaking the thin flanges (4).