  The effect of an external (electro)magnetic field on the finite temperature
transition of QCD is studied. We generate configurations at various values of
the quantized magnetic flux with $N_f=2+1$ flavors of stout smeared staggered
quarks, with physical masses. Thermodynamic observables including the chiral
condensate and susceptibility, and the strange quark number susceptibility are
measured as functions of the field strength. We perform the renormalization of
the studied observables and extrapolate the results to the continuum limit
using $N_t=6,8$ and 10 lattices. We also check for finite volume effects using
various lattice volumes. We find from all of our observables that the
transition temperature $T_c$ significantly decreases with increasing magnetic
field. This is in conflict with various model calculations that predict an
increasing $T_c(B)$. From a finite volume scaling analysis we find that the
analytic crossover that is present at B=0 persists up to our largest magnetic
fields $eB \approx 1 \textmd{GeV}^2$, and that the transition strength
increases mildly up to this $eB\approx1 \textmd{GeV}^2$.
