  We present the discovery of an L subdwarf in 234 square degrees common to the
UK Infrared Telescope (UKIRT) Infrared Deep Sky Survey Large Area Survey Data
Release 2 and the Sloan Digital Sky Survey Data Release 3. This is the fifth L
subdwarf announced to date, the first one identified in the UKIRT Infrared Deep
Sky Survey, and the faintest known. The blue optical and near-infrared colors
of ULAS J135058.86+081506.8 and its overall spectra energy distribution are
similar to the known mid-L subdwarfs. Low-resolution optical (700-1000 nm)
spectroscopy with the Optical System for Imaging and low Resolution Integrated
Spectroscopy spectrograph on the 10.4 m Gran Telescopio de Canarias reveals
that ULAS J135058.86+081506.8 exhibits a strong KI pressure-broadened line at
770 nm and a red slope longward of 800 nm, features characteristics of L-type
dwarfs. From direct comparison with the four known L subdwarfs, we estimate its
spectral type to be sdL4-sdL6 and derive a distance in the interval 94-170 pc.
We provide a rough estimate of the space density for mid-L subdwarfs of
1.5x10^(-4) pc^(-3).
