The invention concerns a one-piece plastic closure (10) for a container (1), the closure consisting of a lower-part (11) and a cap (12) connected to the lower part (11) at a hinge point (13). In the unopened state, a warranty element is held integrally in the cap by rated break points (22). The warranty element has a seal surface in the form of a flat plate (23) with, on its underside, a web (24). The web is held in the warranty position either against the lower part (11) of the closure or against the neck of the container (1). When the cap (12) is opened for the first time, the seal surface (23) is torn out of the top (19) of the cap. This design ensures automatic unsealing without any additional manipulation and permits a large number of embodiments.