Abstract: This stopper (1) comprises a capping lid (20) which, in the back portion of a tubular base (10) for surrounding a container neck, forms part of the upper end (10A) of this base, forming a joint (30) with hinges (31) which defines a geometric axis (A-A) for pivoting the lid between the open and closed service positions. To limit the risk that a child, playing with the stopper, could remove the lid, while at the same time enabling the lid to be opened reliably and satisfactorily by the joint in relation to the base, the joint has two lines of least mechanical resistance (14), which extend from the upper end to the lower end (10B) of the base, being positioned, at the level of said upper end, on both sides of the joint, and which are designed to break before the joint can tear completely when the lid, in open position thereof, is rotated on itself around a geometric axis transverse to the pivoting axis (A-A).