We address the study of gauge invariance in the Standard Model extension which encompasses all Lorentz-violating terms originated by spontaneous symmetry breaking at the Planck scale. In particular, we fully evaluate Ward identities involving two- and three-point functions and derive the conditions which assure gauge invariance of the electromagnetic sector of the Standard Model extension at one loop. We show that momentum routing invariance is sufficient to fix arbitrary and regularization dependent parameters intrinsic to perturbation theory in the diagrams involved. A scheme which judiciously collects finite but undetermined quantum corrections is employed, a particularly subtle issue in the presence of gamma 5 matrices.