  The presence of the warm absorber of considerable optical depth is seen in
many AGN. We show that this medium may affect significantly the optical/UV
spectrum of an AGN by backscattering a fraction of the total radiation flux
towards the disc surface. We consider in detail the case when the disk extends
down to a marginally stable orbit, all the emission comes from the disk surface
and the scattering medium forms a cone around the symmetry axis. Disc
irradiation results in much flatter optical/UV continuum than predicted by
standard disc models. The effect depends both on the total optical depth of the
warm absorber and on the specific density distribution of this medium so the
analysis of the optical/UV continuum allows to obtain constraints for the warm
absorber complementary to those obtained from the soft X-ray data analysis. We
give results for two exemplary sources - RE J1034+396 and PG1211+143, and for
the bluest composite quasar spectrum of Richards et al. obtained from SDSS.
