A closure member comprising a skirt, the skirt extending in a circumferential and an axial direction, a predetermined rupture line extending on the skirt from a first endpoint to a second endpoint, wherein the first and second endpoints are axially offset from each other and the predetermined rupture line extending more than 360° circumferentially so that portions (121, 125) of the predetermined rupture line overlap in the circumferential direction, while the predetermined rupture line comprises a non-overlapping portion, whereby the predetermined rupture line subdivides the closure member into a cap portion, a terminal ring portion and a tether portion defined between the circumferentially overlapping portions of the predetermined rupture line and interposed between the cap portion and the terminal ring portion, wherein the predetermined rupture line has a mechanical strength per unit length which is lower than the mechanical strength per unit length in any other region of the skirt, so that, when a tensile force is applied between the cap portion and the terminal ring portion, the closure member ruptures at the predetermined rupture line, wherein the predetermined rupture line has a first circumferential portion extending from the first endpoint of the predetermined rupture line adjacent the cap portion, and a second circumferential portion extending from the first circumferential portion to the second endpoint of the predetermined rupture line adjacent the terminal ring portion, wherein the mechanical strength per unit length in the first circumferential portion of the predetermined rupture line is smaller than the mechanical strength per unit length in the second circumferential portion of the predetermined rupture line; characterized in that the first circumferential portion extends 360 degrees circumferentially, and in that the non-overlapping portion of the predetermined rupture line is comprised in the first circumferential portion of the predetermined rupture line. a predetermined rupture line extending on the skirt from a first endpoint to a second endpoint, wherein the first and second endpoints are axially offset from each other and the predetermined rupture line extending more than 360° circumferentially so that portions (121, 125) of the predetermined rupture line overlap in the circumferential direction, while the predetermined rupture line comprises a non-overlapping portion, whereby the predetermined rupture line subdivides the closure member into a cap portion, a terminal ring portion and a tether portion defined between the circumferentially overlapping portions of the predetermined rupture line and interposed between the cap portion and the terminal ring portion, wherein the predetermined rupture line has a mechanical strength per unit length which is lower than the mechanical strength per unit length in any other region of the skirt, so that, when a tensile force is applied between the cap portion and the terminal ring portion, the closure member ruptures at the predetermined rupture line, wherein the predetermined rupture line has a first circumferential portion extending from the first endpoint of the predetermined rupture line adjacent the cap portion, and a second circumferential portion extending from the first circumferential portion to the second endpoint of the predetermined rupture line adjacent the terminal ring portion, wherein the mechanical strength per unit length in the first circumferential portion of the predetermined rupture line is smaller than the mechanical strength per unit length in the second circumferential portion of the predetermined rupture line; characterized in that the first circumferential portion extends 360 degrees circumferentially, and in that the non-overlapping portion of the predetermined rupture line is comprised in the first circumferential portion of the predetermined rupture line.