In this contribution, we will present the results of an ambitious program aimed at discovering and characterising metal-poor low-mass stars and brown dwarfs, nicknamed ultracool subdwarfs. On the one hand we are running a spectroscopic follow-up with GTC and VLT of subdwarf candidates identified in large-scale surveys (UKIDSS, SDSS, and 2MASS). On the other, we have obtained high-resolution optical spectroscopy with the VPH gratings on GTC/OSIRIS to look for the lithium line in a subdwarf located at the hydrogen-burning limit in order to constrain its mass.