Deviations from the Paschen's law in air and nitrogen are investigated for gaps from 100 nm to 10 μm, by using a high precision electrode positioning system. The deviation is observed when electrode gaps are smaller than 4 μm at atmospheric pressure. At distances lower than 1 μm, a nearly constant average breakdown field of 350 V/μm is evidenced in both gases with Au and Ru electrodes. A metallic plasma initiated by field emission from the cathode can explain the reduction of the breakdown voltage at such low gaps. In ambient air, the existence of a pre-breakdown current is also evidenced, probably due to the presence of water adsorbed on electrodes.