  We consider a planar SIS-type Josephson junction between diffusive
superconductors (S) through an insulating tunnel interface (I). We construct
fully self-consistent perturbation theory with respect to the interface
conductance. As a result, we find correction to the first Josephson harmonic
and calculate the second Josephson harmonic. At arbitrary temperatures, we
correct previous results for the nonsinusoidal current-phase relation in
Josephson tunnel junctions, which were obtained with the help of conjectured
form of solution. Our perturbation theory also describes the difference between
the phases of the order parameter and of the anomalous Green functions.
