The stability of plasma in a magnetic field against collision‐free tearing of current layers is considered in plane geometry. While previous studies were confined to specific equilibria without charge separation, the present (variational) approach allows for a large class of distribution functions and for an equilibrium electrostatic field, thus also applying to strongly inhomogeneous equilibria. A simple criterion is obtained, which is necessary and sufficient for stability against perturbations that do not vary along the direction of the equilibrium current (tearing mode). Strictly one‐dimensional perturbations requiring a somewhat modified procedure are discussed separately.