  We investigate the effects of strong magnetic fields on the equation of state
of warm stellar matter as it may occur in a protoneutron star. Both neutrino
free and neutrino trapped matter at a fixed entropy per baryon are analyzed. A
relativistic mean field nuclear model, including the possibility of hyperon
formation, is considered. A density dependent magnetic field with the magnitude
$10^{15}$ G at the surface and not more than $3\times 10^{18}$ G at the center
is considered. The magnetic field gives rise to a neutrino suppression, mainly
at low densities, in matter with trapped neutrinos. It is shown that an hybrid
protoneutron star will not evolve to a low mass blackhole if the magnetic field
is strong enough and the magnetic field does not decay. However, the decay of
the magnetic field after cooling may give rise to the formation of a low mass
blackhole.
