Abstract Torus instability is currently considered the most likely mechanism for the sudden onset of the rapid rise of magnetic flux ropes observed on the Sun as prominence eruptions and coronal mass ejections. However, the shape of flux ropes containing prominence material is far from perfectly circular. To understand the instability threshold for real structures in the solar atmosphere, it is necessary to consider more appropriate flux-rope shapes. We calculated the critical decay index n c for a rectangular circuit model of an eruptive prominence and found that it is below unity for most rectangular aspect ratios. Such low critical decay indices are considered extremely problematic for the standard theory of torus instability.