We study the stability of an infinite differentially heated liquid metal layer bounded by two horizontal walls and submitted to an external magnetic field subjected to a slight polar deviation with respect to the initially selected direction. The effects of the weak deviation on the stability characteristics of the two-dimensional stationary and three-dimensional oscillatory instabilities which can develop in such a liquid layer are investigated. The flow exhibits some interesting and unexpected stability characteristics. The effect of the deviation is either stabilizing or destabilizing depending essentially on the initially selected magnetic field orientation: longitudinal, transverse or vertical. In general, when the deviation angle increases the critical values of the Grashof number, the wave numbers and the frequency deviate from the asymptotic behaviours observed for large values of the Hartmann number, Ha > 20, in the case without deviation. New features and behaviours are found for some orientations of the magnetic field. (c) 2007 Elsevier B.V. All rights reserved.