... These sources are much more likely to be gravitationally lensed by intervening galaxies as compared with sources discovered in surveys in other wavebands. ... The effect of lensing by clusters is then discussed briefly. This is expected to increase the source confusion noise in a Planck survey in the directions of clusters, and could provide information about galaxies with flux densities that are too faint to be detected directly by Planck . ... 2 The properties of galaxy--galaxy lensing . ...