The stopper (1) has a tubular skirt (12) for fastening it around the container neck, an end wall (10) perpendicular to the central axis (X-X) of the skirt, and an annular sealing lip (16) extending into the skirt from the inner face (10A) of the end wall. A protrusion (22) connects the inner face of the end wall to the inner face (16B) of a peripheral portion of the lip in order to transmit a movement of the end wall to the lip in the event of excess pressure in the container. In order to release this excess pressure reliably and in a manner which is easy to carry out, the protrusion occupies all the space delimited jointly by the inner face of the end wall, by the inner face of said lip portion and by a geometric plane (P) which passes through said lip portion and intersects the inner face of the end wall along a straight line (L) orthoradial to the axis of the skirt.