  The Planck sub-mm surveys detected the brightest strongly gravitationally
lensed dusty galaxies in the sky. The combination of their extreme
gravitational flux boosting and image stretching offers the unique possibility
of measuring in detail, via high-resolution imaging and spectroscopic
follow-up, the galaxy structure and kinematics in early evolutionary phases,
thus gaining otherwise unaccessible direct information on physical processes in
action. The extraction of candidate strongly lensed galaxies (SLGs) from Planck
catalogues is hindered by the fact that they are generally detected with poor
S/N, except for the few brightest ones, their photometric properties are
strongly blurred and they are difficult to single out. We devised a method to
increase by a factor of 3 to 4 the number of identified Planck-detected SLGs,
although with an unavoidably limited efficiency. Our approach uses the fact
that SLGs have sub-mm colours colder than nearby dusty galaxies (the large
majority of Planck extragalactic sources). The sub-mm colours of the 47
confirmed or very likely Planck-detected SLGs are used to estimate the colour
range of these objects. Moreover, most nearby galaxies and radio sources can be
picked up by cross-matching with IRAS and PCNT catalogues, respectively. We
present samples of 177, 97, 104 lensed candidates at 545, 857, 353 GHz,
respectively. The efficiency of our approach, tested on the SPT survey covering
2,500 sq. deg., is estimated to be of 30%-40%. We also discuss stricter
selection criteria increasing efficiency to 50% but with a somewhat lower
completeness. Our analysis of SPT data has identified a dozen of galaxies that
can be reliably considered previously unrecognized Planck-detected SLGs.
Extrapolating the number of Planck-detected confirmed or very likely SLGs found
within the SPT and H-ATLAS areas, we expect from 150 to 190 such sources over
the|b|>20deg sky.
