Stimulated Brillouin backscattering (SBBS) of an extraordinary electro- magnetic wave from a magnetized homogeneous plasma is investigated. A system of equations that describes the problem is derived and solved for a weak-field limit. A modified expression for the maximum growth rate in magnetized plasmas is derived, which recovers the growth rate expression for non-magnetized plasma known in literature. Small values of static magnetic field are found to increase the instability growth rate, while high magnetic fields reduce the instability bringing it to zero at a cut-off field, beyond which the plasma becomes stable, a result that is consistent with numerical results known in the literature (Bawa'aneh, M. S. 2003 Contrib. Plasma Phys. 43, 447). Also, a threshold for incident laser intensity is found, below which the SBBS maximum growth rate is suppressed. The value of this threshold is found to increase as the static magnetic field increases.