The invention relates to a magnetic tape cassette and a tape recorder for use with such a cassette. More specifically, the invention relates to the provision of movable holding means within the cassette, which means -- in a first position -- prevent unwinding of the packs of tape and thus hold the magnetic tape taut and -- in a second position -- release the packs and the magnetic tape for operation in conjunction with guide, scanning and driving means on the recorder. The said holding means are in the form of spring elements which engage teeth in the hubs to prevent the packs from unwinding. In a special embodiment, the holding means are in the form of flat members, one on each side of the packs between the side surfaces thereof and the upper and lower walls of the cassette, individual springs being provided which resiliently support said members relative to the wall of the cassette and permit movement of said members from a first, pack-locking position to a second, pack-releasing position.