Electric field measurements made on board the four Cluster II satellites are used to estimate the spatial scale of the electrostatic potential in the front of the terrestrial bow shock in the quasi-perpendicular regime. High-amplitude electric fields (up to 70 mV m-1) were measured at the shock fronts encountered on 31 March 2001. The main change in the cross-shock potential took place on spatial scales of a few electron inertial lengths. Such small scales in the electrostatic potential can be explained in the framework of the quasi-electrostatic subshock concept. The high magnitude of the upstream magnetic field, which resulted in an unusually small ¿ and moderate Mach numbers, created favorable conditions for the occurrence of quasi-electrostatic subshocks in the shock front.