Abstract In this paper a stability analysis is carried out for a liquid metal film flowing over an inclined nonconducting chute with coplanar toroidal magnetic field. A successive expansion technique is employed to investigate the nature and growth rate of the MHD instability caused by the long wavelength perturbations. In the unmagnetized limit, the derived result reduces to the well-known stability criterion for the non-conducting fluid case. It is found that for a sufficiently thick, moderately fast film, the stability criterion can be satisfied. However, if considerably higher flow speed is required (for example, to avoid eruptions of hydrogen bubbles formed during the film's exposure to the charged particle bombardment) then the flow can be MHD-wise unstable. Chutes of very narrow width have to be employed in order to achieve stability, which may not be structurally desirable.