A capping device includes an external outer cap adapted to be removably fixed to the exterior face of a container neck, and an internal insert adapted to plug the opening of the neck. The insert is adapted to be fitted to the neck independently of the outer cap and before fitting the outer cap to the neck. When it is fitted to the neck, the outer cap is adapted to be permanently fastened to the insert. When filling the container, the insert may include means for retaining it on the neck adapted to connect the insert mechanically to the neck before the outer cap is fitted to the neck, thereby sealing at least the exterior peripheral surface of the free end of the neck against a cleaning liquid applied externally to the neck.