  We propose a model-independent parametrization for the one-pion-to-vacuum
matrix elements of the vector and axial vector hadronic currents in the
presence of an external uniform magnetic field. It is shown that, in general,
these hadronic matrix elements can be written in terms of several gauge
covariant Lorentz structures and form factors. Within this framework we obtain
a general expression for the weak decay $\pi^- \to l\,\bar\nu_l$ and discuss
the corresponding limits of strong and weak external magnetic fields.
