  We aim at developing an efficient method to search for late-type subdwarfs
(metal-depleted dwarfs with spectral types >M5) to improve the current
statistics. Our objectives are: improve our knowledge of metal-poor low-mass
dwarfs, bridge the gap between the late-M and L types, determine their surface
density, and understand the impact of metallicity on the stellar and substellar
mass function.
  We carried out a search cross-matching the SDSS, 2MASS, and UKIDSS using
STILTS, Aladin, and Topcat. We considered different photometric and proper
motion criteria for our selection. We identified 29 and 71 late-type subdwarf
candidates in each cross-correlation over 8826 and 3679 square degrees,
respectively. We obtained low-resolution optical spectra for 71 of our
candidates with GTC, NOT, and VLT and retrieved spectra for 30 candidates from
the SDSS spectroscopic database. We classified 92 candidates based on 101
optical spectra using two methods: spectral indices and comparison with
templates of known subdwarfs.
  We confirmed 86% and 94% of the candidates as late-type subdwarfs from the
SDSS vs 2MASS and SDSS vs UKIDSS cross-matches, respectively. These subdwarfs
have spectral types ranging between M5 and L0.5 and SDSS magnitudes in the
r=19.4-23.3 mag range. Our new late-type M discoveries include 49 subdwarfs, 25
extreme subdwarfs, six ultrasubdwarfs, one subdwarf/extreme subdwarf, and two
dwarfs/subdwarfs. We derived a surface density of late-type subdwarfs of
0.040$^{+0.012}_{-0.007}$ per square degree in the SDSS DR7 vs UKIDSS LAS DR10
cross-match. We also checked the AllWISE photometry of known and new subdwarfs
and found that mid-infrared colours of M subdwarfs do not appear to differ from
their solar-metallicity counterparts of similar spectral types. However, the
J-W2 and J-W1 colours are bluer for lower metallicity dwarfs. (abstract
strongly abridged)
