  We investigate the stability of the topological phase of the toric code model
in the presence of a uniform magnetic field by means of variational and
high-order series expansion approaches. We find that when this perturbation is
strong enough, the system undergoes a topological phase transition whose first-
or second-order nature depends on the field orientation. When this transition
is of second order, it is in the Ising universality class except for a special
line on which the critical exponent driving the closure of the gap varies
continuously, unveiling a new topological universality class.
