A pre-assembled container closure assembly (1) has a plastic spout body (2) with a tubular neck (4) delimiting a product passage (6) that extends to a mouth opening (8) of the neck. A rotational cap (30) is secured in closed position on the neck. The cap has a cap body including a top wall (31)and a skirt (32). The cap body further includes an annular inner sealing ring (40)depending from the top wall. The inside of the neck is provided with an inward tapering annular guide surface (15) that is contacted by the inner sealing ring when the cap is secured onto the spout body and then causes an elastic deformation of the inner sealing ring and/or the neck. The inner sealing ring is embodied such that the first annular sealing surface (41) on the exterior side of the inner sealing ring and a lowermost annular surface (43) of the interior side of the sealing ring adjoin one another directly at an apex (44) which forms the lower edge of the inner sealing ring, and in that - with the cap in closed position - said apex lies against the interior side of the neck, said lowermost annular surface extending from said apex to form an angle of at least 90° with an exposed surface portion of the interior side of the neck directly below said apex so as to avoid the presence of any gap between the inner sealing ring and the neck at said apex.