We investigate inviscid instability in an electrically conducting fluid affected by a parallel magnetic field. The case of low magnetic Reynolds number in Poiseuille flow is considered. When the magnetic field is sufficiently strong, for a flow with low hydrodynamic Reynolds number, it is already known that the neutral disturbances are three-dimensional. Our investigation shows that at high hydrodynamic Reynolds number(inviscid flow), the effect of the strength of the magnetic field on the fastest growing perturbations is limited to a decrease of their oblique angle i.e. angle between the direction of the wave propagation and the basic flow. The waveform remains unchanged. The detailed analysis of the linear instability provided by the eigenvalue problem shows that the magnetic field has a stabilizing effect on the electrically conducting fluid flow. We find also that at least, the unstability appears if the main flow possesses an inflexion point with a suitable condition between the velocity of the basic flow and the complex stability parameter according to Rayleigh's inflexion point theorem.