The invention relates to a container closure made of plastic, comprising a lower closure part, which can be fastened to an opening of a container. Of the invention, the lower closure part has a collar, which can be inserted into the opening of the container. The at least one locking projection can be moved in the direction of a center axis of the lower closure part in a reversible elastic manner in relation to the collar. The at least one locking projection is designed in such a way that, when the at least one locking projection and an undercut shaped in the opening engage with each other, the at least one locking projection and said undercut form a lock that cannot be detached in the direction of the center axis. The collar has at least one through-hole, and the locking projection can be displaced through the through-hole.