  In this work we investigate the effect of an external magnetic field B on the
shape of flux tubes in QCD by means of lattice simulations, performed with
N_f=2+1 flavors of stout improved dynamical staggered quarks with physical
masses. After having discussed some difficulties in the practical definition of
the flux tube at B=0, we show that these ambiguities do not affect the
determination of the flux tube modifications induced by the magnetic field.
Different results are obtained depending on the relative orientations of the
flux tube and of the magnetic field: they confirm that the magnetic field acts
as transverse confinement catalyser and longitudinal confinement inhibitor;
moreover, the flux tube itself loses its axial symmetry when it is not directed
along the magnetic background.
