  We calculate the subgap current in planar superconducting tunnel junctions
with thin-film diffusive leads. It is found that the subharmonic gap structure
of the tunnel current scales with an effective tunneling transparency which may
exceed the junction transparency by up to two orders of magnitude depending on
the junction geometry and the ratio between the coherence length and the
elastic scattering length. These results provide an alternative explanation of
anomalously high values of the subgap current in tunnelling experiments often
ascribed to imperfection of the insulating layer. We also discuss the effect of
finite lifetime of quasiparticles as the possible origin of additional
enhancement of multiparticle tunnel currents.
