Abstract: A closing cap molded in one piece from a plastics material for sealing a container with a threaded neck has a top plate (3), an annular skirt (4) with an internal screw thread (5) adapted to cooperate with the threaded container neck (2), and an integrally molded deformable annular sealing ring (6) extending downwardly from an inner surface of said top plate (3) and spaced radially inwardly of the annular skirt (4), wherein when the closing cap (1) is screwed onto the container neck (2), the sealing ring (6) is deformed outwardly and is fully conformed to the outer surface of the container neck (2) without being clamped between the container neck (2) and the annular skirt (4) and without being pressed against the annular skirt (4).