The T2K near detector (ND280) located 280 m down the neutrino beam at the J-PARC accelerator is currently being upgraded and equipped with new sub-detectors, including a Super Fine-Grained Scintillator detector (sFGD) and two High-Angle Time Projection Chambers (HA-TPCs). The HA-TPCs operate at atmospheric pressure with the “T2K gas” mixture (Ar:CF4:iC4H10=95:3:2) contained in a Field Cage with a central cathode splitting the active volume (2.0×1.8×0.8 m^3) into two halves (1m long in drift direction). The Field Cages with 3-cm-thin walls are built by exploiting composite material techniques with lightweight and low-Z materials. Specific building processes were developed for complying with very tight geometrical tolerances and several requirements including very low material budget, high degree of electrical field uniformity, adequate degrading of high electric potential (the cathode is operated at -30kV) along very reduced distances to electrical ground, high structural integrity against over-pressure, gravitational and thermal loads, minimum outgassing from inner surfaces, minimum permeability to atmospheric gas components having a negative impact on electron drift (O2, N2 and H2O). The Cage innermost layer consists of a Kapton foil with copper strips on both sides ( “Field” and “Mirror” strips), which allow degrading the electric potential along the drift volume with high degree of uniformity of the resulting Electric Field up to few mm from the surface. This Poster focuses on the extensive mechanical and electrical characterization of the Field Cages, including assessment of mechanical properties, HV dielectric insulation systematic tests, studies about inner surfaces characteristics and internal E-field uniformity. Concerning E-field we accurately assessed its uniformity by exploiting different techniques including (a) measurement of straight tracks produced by laser beams injected inside the TPC and (b) direct measurement of the E-field components inside the Field Cage by means of a fully dielectric (no metallic parts) custom developed device based on the Pockels effect.