Metal-poor ultracool dwarfs (ultracool subdwarfs, including very-low-mass stars and brown dwarfs) are tracers of the pristine chemical composition at early phases in the assembly of the Milky Way. They are poorly understood because they are extremely faint and rare. Our aims are to improve the knowledge of ultracool subdwarf candidates, to break the degeneracy between metallicity and temperature and to investigate their atmospheric properties, and to increase the discovery space to larger galactocentric distances than ever before. Deep z-band images and low-resolution optical spectra of ultracool subdwarf candidates were collected with the 10.4-meter Gran Telescopio de Canarias (GTC) at La Palma, Spain and 8.2-meter Very Large Telescope (VLT) at Cerro Paranal, Chile. Photometric measurements of the z-band flux were performed for these ultracool subdwarfs and they were combined with infrared photometry in J, H, K, W1, and W2 bands from the literature to obtain the colours. The trigonometric parallaxes of the brightest T subdwarfs have been measured using the 3.5-m telescope at Calar Alto, Spain, yielding accurate distances. Using this information as reference, photometric selection criteria have been defined to select ultracool subdwarf candidates in Euclid Early Release Observations of high galactic latitude fields. The first results from this work will be presented at this conference.