  We study the effect of strong magnetic fields, of the order of
$10^{15}-10^{17}$ G, on the extension of the crust of magnetized neutron stars.
The dynamical instability region of neutron-proton-electron ($npe$) matter at
subsaturation densities and the mode with the largest growth rate are
determined within a relativistic mean field model. It is shown that a strong
magnetic field has a large effect on the instability region, defining the
crust-core transition as a wide density range.
