Primeval very low-mass stars and brown dwarfs - V. A halo L3 subdwarf with prograde eccentric orbit in the Galactic plane

VVV J12564163-6202039 is an L subdwarf located in the Galactic plane (b = 0{^{circ}_.}831) discovered by its high proper motion (1.1 arcsec yr^-1). We obtained an optical to near-infrared spectrum of it with the X-shooter on the Very Large Telescope, and re-classified it as an sdL3 subdwarf. Its best-fitting BT-Dusty model spectrum has T_eff = 2220 K, [Fe/H] = -0.9, and log g = 5.5. It is just below the stellar/substellar boundary in the T_eff versus [Fe/H] parameter space. This object is at a distance of 66.94^{+8.49}_{-6.77} pc according to the Gaia astrometry. It has a halo membership probability of 99.82 per cent according to its Galactic kinematics. It has unusually flat (|Z| ≲500 pc) and extremely eccentric (e = 0.9) prograde orbit that takes the source from as close as 1.2 kpc from the Galactic centre to 24 kpc out. VVV J12564163-6202039 joined by SDSS J133348.24+273508.8 and ULAS J021258.08+064115.9 are the only three known objects in the sdL subclass that have halo kinematics.