The claimed stopper (1) with a warranty strip (6) for a container such as a flask or bottle comprises a section of a cylindrical wall (2) surrounding the aperture of the container for which the stopper is intended, the wall section in question being formed on its outer face and in the vicinity of the edge as an uninterrupted circular outer beading (3). The stopper (1) comprises: a section of the cap (4) with a diameter greater than the outer diameter of the said section of the wall (2) with the outer beading (3), this section of the cap (4) being a ring-shaped part (5) and situated so as to surround the said section of the wall (2); and a warranty strip (6) arranged in the extension of the said ring-shaped part (5) and provided with a circular uninterrupted or interrupted retaining device (7), e.g. tabs. The retaining device (7) together with the outer beading (3) impedes the passage of the warranty strip (6) across said outer beading (3) in the direction of the container aperture. The proposed stopper is characterised by the fact the warranty strip (6) and retaining device (7), on the one hand, and the section of the cap (4) and ring-shaped part (5), on the other, are separate parts which are joined together so as to be detachable.