The region of instability for planar reaction fronts of cubic autocatalysis between ionic species under constant electric field has been determined accurately. The ratio of diffusion coefficients at the onset of instability delta(cr) is substantially varied by the component-dependent drift and directly proportional to the concentration of the autocatalyst behind the front beta(s) as delta(cr)=2.3002 beta(s). This opens the possibility to use electric field as a control parameter for reaction-front instabilities. The dispersion relation calculated from the linear stability analysis of the full system is in good agreement with the initial evolution of the Fourier modes associated with the slightly perturbed planar reaction front obtained by the direct integration of the governing equations in two spatial dimensions.