A degenerate neutron gas in equilibrium with a background of electrons and protons in a magnetic field exerts its pressure anisotropically, having a smaller value perpendicular than along the magnetic field. For critical fields the magnetic pressure may produce the vanishing of the equatorial pressure of the neutron gas, and the outcome could be a transverse collapse of the star. This fixes a limit to the fields to be observable in stable pulsars as a function of their density. The final structure left over after the implosion might be a mixed phase of nucleons and meson ($π^{\pm,0},κ^{\pm,0}$) condensate (a strange star also likely) or a black string, but no magnetar at all.