The full-wave spectral domain approach together with the EBM method is used to analyse the nonreciprocal wave propagation in laterally open boundary planar structures with a multilayered gyroelectric substrate. A study of the nonreciprocal surface slow waves appearing in an externally magnetised MIS line is presented. This work also presents the ability of that configuration to support both slow wave and nonreciprocal effects with reasonable losses. Various numerical results are presented that show the dependence of the propagation parameters on the magnitude and orientation of the biasing magnetic field. It is shown that external control of the magnitude of the biasing field permits equalisation of the modal phase velocities in a pair of coupled strips on a magnetised MIS configuration.