Recent simulations of JET and ASDEX Upgrade plasmas with EDGE2D and SOLPS, respectively, showed that upstream radial electric field in the Scrape-Off Layer (SOL) is substantially below values expected from simple estimates, based on the effects of the potential Debye sheath drop at the target and thermoelectric force. This served as a motivation for the dedicated EDGE2D-Nimbus modelling of JET plasmas aimed at identification of key mechanisms responsible for the formation of the radial electric field (Er) in the main SOL of high recycling divertor plasmas. Code runs with different upstream density and input power levels were carried out, aimed at obtaining both 'cold' (with flat outer target electron temperature, Te, profiles and low peak Te values, compared to the upstream ones) and 'hot' (with peaked outer target Te profiles and the maximum target Te values comparable to the upstream ones) divertor solutions. It was found that in 'cold' divertor solutions, the contribution of target Debye sheath drops to the formation of the upstream Er is small, as expected. Under such conditions, other contributions, originating from the parallel electron force balance, become important. The two main contributions: from the thermoelectric force (-0.71∇||Te, for singly charged ions), and parallel electron pressure gradient (-∇||pe/ne ), were found, however, to counteract each other, reducing the upstream Er. In 'hot' divertor solutions, the Debye sheath mechanism is more important, but it is partly compensated by the (radial) profile effects of the increased -∇||pe/ne contribution (pe and ne-electron pressure and density). The latter is related to the formation of a local maximum of pe around the X-point position on flux surfaces close to the separatrix and is attributed to ionization of neutrals.