In this paper we investigate the effect of a uniform vertical magnetic field on the linear growth land decay) rates of steady Manngoni convection in a horizontal layer of electrically-conducting fluid heated from below. Explicit analytical expressions for the linear growth rates of both long- and short-wave instability modes are derived for the first time. Numerically-calculated results for the linear growth rates are also presented. In particular, we show that the effect of increasing the magnetic field strength is always to stabilise the layer by decreasing the growth rates of the unstable modes.