The invention relates to a screw cap for a container neck, comprising an internally threaded cap portion and a security ring joined by means of inclined bridge portions to the cap portion. The sealing ring is resiliently deformable and is designed to be forced over and past a collar around the neck of the container, when the cap is being screwed on for the first time. Recesses are provided in the lower part of the cap portion and/or the upper part of the sealing ring, in which recesses the bridge portions are secured. The bridge portions are so inclined that the upper edge of the security ring can engage the lower edge of the cap skirt when, during screwing on of the cap for the first time, the security ring engages the collar, without undue bending of the bridge portions, so that the cap portion can support the security ring while the latter is forced over the collar. However, when the cap is unscrewed, and the security ring is stopped by the collar, the bridge portions are first bent substantially, and thereby weakened and are tensioned, to rupture, during continued unscrewing of the cap portion.