  The aim of the project is to improve our knowledge on the low-mass and
low-metallicity population to investigate the influence of metallicity of the
stellar (and substellar) mass function.
  We present the results of a photometric and proper motion search aimed at
unearthing ultracool subdwarfs in large-scale surveys. We employed and combined
the UKIDSS LAS DR5 and the SDSS DR7 complemented with ancillary data from
2MASS, DENIS and SuperCOSMOS.
  The SDSS DR7 vs UKIDSS LAS DR5 search returned a total of 32 ultracool
subdwarf candidates, only two being recognised as a subdwarf in the literature.
Twenty-seven candidates were followed-up spectroscopically in the optical
between 600 and 1000 nm. We confirmed 20 candidates as subdwarfs, extreme
subdwarfs or ultra-subdwarfs with spectral types later than M5; this represents
a success rate of ~60%. Among those 20 new subdwarfs, we identified 2 early-L
subdwarfs very likely located within 100 pc that we propose as templates for
future searches because they are the first examples of their subclass. Another
7 sources are solar-metallicity M dwarfs with spectral types between M4 and M7
without Halpha emission, suggesting that they are old M dwarfs. The remaining 5
candidates do not have spectroscopic follow-up yet; only 1 remains as a
bona-fide ultracool subdwarf after revision of their proper motions. We
assigned spectral types based on the current classification schemes and, when
possible, we measured their radial velocities. Using the limited number of
subdwarfs with trigonometric parallaxes, we estimated distances between 90 and
600 for the new subdwarfs. We provide mid-infrared photometry from WISE for two
subdwarfs and discuss their colours. Finally, we estimate a lower limit of the
surface density of ultracool subdwarfs of the order of 5000-5700 times lower
than that of solar-metallicity late-M dwarfs (Shortened).
