Tank filler neck closure for a fuel tank, having a device for protecting against incorrect fuelling, said device being arranged in a tube filler neck of a tank filling tube and having a closure flap and a locking mechanism, which can be unlocked by means of a nozzle tube for fuel, for the closure flap, wherein the locking mechanism comprises: - a release slide which can be moved transversely to the axial direction of the tube filler neck from a locking position into an unlocking position and which has a passage for the nozzle tube to the closure flap placed behind the slide in the insertion direction of said slide, - at least one securing device that can be actuated in the axial direction of the tube filler neck by the nozzle tube, and, in a first position in which the securing device is not actuated by the nozzle tube, locks the release slide in the release slide locking position and, in a second position in which the securing device is actuated by the nozzle tube in the axial direction, releases the release slide in order to move same into the unlocking position, wherein the securing device has at least one latching element with a stop surface which extends into the passage substantially transversely to the insertion direction in the release slide in a radial direction and which is adapted to the cross-section of at least two permissible nozzle tubes (14a, 14b) with different thicknesses, wherein a passage cross-section which is defined by the free end of the at least one stop surface protruding into the passage and by an internal circumferential region of the passage is a little smaller than the cross-section of the thinnest permissible nozzle tube in the locking position of the release slide, and the passage cross-section is at least as large as the cross-section of the largest permissible nozzle tube in the unlocking position, characterised in that the stop surface on the at least one latching element is such that, in the locking position, it can be contacted both by a nozzle tube with a larger permissible tube diameter and by such a nozzle tube with a smallest permissible tube diameter, wherein the stop surface (n) and the release slide are simultaneously acted upon by the nozzle tube at least at the beginning of the displacement path thereof from the locking position and the nozzle tube with both the larger and the smallest permissible diameter is out of operative engagement with the stop surface of each latching element at least when the release slide is in the unlocking position. - a release slide which can be moved transversely to the axial direction of the tube filler neck from a locking position into an unlocking position and which has a passage for the nozzle tube to the closure flap placed behind the slide in the insertion direction of said slide, - at least one securing device that can be actuated in the axial direction of the tube filler neck by the nozzle tube, and, in a first position in which the securing device is not actuated by the nozzle tube, locks the release slide in the release slide locking position and, in a second position in which the securing device is actuated by the nozzle tube in the axial direction, releases the release slide in order to move same into the unlocking position, wherein the securing device has at least one latching element with a stop surface which extends into the passage substantially transversely to the insertion direction in the release slide in a radial direction and which is adapted to the cross-section of at least two permissible nozzle tubes (14a, 14b) with different thicknesses, wherein a passage cross-section which is defined by the free end of the at least one stop surface protruding into the passage and by an internal circumferential region of the passage is a little smaller than the cross-section of the thinnest permissible nozzle tube in the locking position of the release slide, and the passage cross-section is at least as large as the cross-section of the largest permissible nozzle tube in the unlocking position, characterised in that the stop surface on the at least one latching element is such that, in the locking position, it can be contacted both by a nozzle tube with a larger permissible tube diameter and by such a nozzle tube with a smallest permissible tube diameter, wherein the stop surface (n) and the release slide are simultaneously acted upon by the nozzle tube at least at the beginning of the displacement path thereof from the locking position and the nozzle tube with both the larger and the smallest permissible diameter is out of operative engagement with the stop surface of each latching element at least when the release slide is in the unlocking position.