A closure system (3) for closing an opening (25) of a container (2) comprises a stopper member (31), a cage member (32) and a lock activation formation (34). The stopper member (31) is made of an elastic material and has a plug portion (311) configured to fit into the opening (25) of the container (2) and a cover portion (312) configured to abut a boundary surface (24) adjacent to the opening (25) of the container (2). The cage member (32) is made of a shape-retentive material. The closure system (3) is configured to be in an assembled state in which the plug portion (311) of the stopper member (31) is fitted into the opening (25) of the container (2), the cover portion (312) of the stopper member (31) abuts the boundary surface (24) of the container (2) and the cage member (32) is mounted to the container (2). The cage member (32) has a locking structure (322), wherein the locking structure (322) of the cage member (32) is configured, in the assembled state of the closure system (3), to be in a detached position in which the locking structure (322) of the cage member (32) is disengaged from the container (2), and in a fixed position in which the locking structure (322) engages a corresponding structure of the container (2). The lock activation formation (341, 342) is configured to interact with the cage member (32) such that the cage member (32) changes from the detached position to the fixed position.