A pre-assembled container closure assembly adapted to be secured onto a container, said assembly comprising: - a monolithic injection moulded spout body of thermoplastic material, said spout body having at a lower end thereof a base portion that is adapted to secure the assembly onto a container and said body having at an upper end thereof a tubular neck extending in an axial direction thereof, which neck has an interior side delimiting at least a section of a product passage that extends from a lower product passage opening through the spout body to a mouth opening at a top end of the neck, said tubular neck having an exterior side, - a manually removable rotational cap that is secured in closed position thereof on said tubular neck, the cap in said closed position forming the sole closure of the product passage through the spout body and the cap being manually removable by rotation to open the product passage, wherein the cap has a cap body that is monolithic injection moulded of thermoplastic material, which cap body includes a top wall that extends in closed position across said mouth opening, and which cap body includes a downward annular skirt depending from said top wall, said skirt having an interior side, an exterior side and a lower edge, wherein the exterior side of the neck and the interior side of the skirt each have integrally moulded and cooperating rotational connection means, preferably first and second screw threads (10, 33) on said neck and skirt respectively, wherein the cap body further includes an integrally moulded annular inner sealing ring depending from said top wall, concentric with respect to the skirt, the inner sealing ring having an interior side, an exterior side, and a lower edge, wherein the exterior side of the sealing ring includes a first annular sealing surface that-with the cap in closed position - is in sealing contact with a corresponding first annular sealing surface of the interior side of the neck to provide a first seal between the cap and the spout body, wherein the exterior side of the neck at or near the top end thereof forms a second annular sealing surface that - with the cap in closed position - is in sealing contact on a corresponding second annular sealing surface of the cap to provide a second seal between the cap and the spout body, characterized in that the interior side of the neck is provided with an inward tapering annular guide surface that extends from said mouth opening downward and 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, and in that the inner sealing ring is embodied such that the first annular sealing surface on the exterior side of the inner sealing ring and a lowermost annular surface of the interior side of the sealing ring adjoin one another directly at an apex 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. - a monolithic injection moulded spout body of thermoplastic material, said spout body having at a lower end thereof a base portion that is adapted to secure the assembly onto a container and said body having at an upper end thereof a tubular neck extending in an axial direction thereof, - a manually removable rotational cap that is secured in closed position thereof on said tubular neck, the cap in said closed position forming the sole closure of the product passage through the spout body and the cap being manually removable by rotation to open the product passage,