The invention concerns a plastic closure for fitting on to the neck of a liquid container in positively locking and sealed relationship comprising a head plate (1) provided for covering a container opening and an outer seal (7) which is circumferential along the edge of the head plate (1) and is provided for engagement with the outside of a container neck (20) defining the container opening, and an inner seal (5) which is circumferential within the outer seal (7) and parallel thereto and which is provided for engagement with the inside of the container neck, wherein the outer and the inner seals (7, 5) respectively extend approximately perpendicularly or slightly inclined from an inside (11) of the head plate (1). To provide a closure and also a container having a closure which generally prevent the closure being blown off under a high positive pressure it is proposed according to the invention that the outer seal (7) has a weakened region (6, 6′, 6″) in at least one circumferential portion so that the outer seal (7) becomes unsealed at a lower internal positive pressure than when the outer seal is of a configuration without the weakened region.