Research programs for the development and experimental estimation of diagnostics are under way at Joint European Torus (JET) - one of the largest operating fusion reactors. These diagnostics are relevant for solving diagnostic tasks of International Thermonuclear Experimental Reactor (ITER). The paper discusses the principles of designing the magnetic measurement instrumentation based on the application of steady-state radiation-resistant semiconductor Hall sensors. It also presents the results of the instrumentation's application for ex-vessel magnetic field measurement on JET reactor during the latest deuterium campaign, and the prospects of further enhancement in magnetic diagnostic accuracy and reliability. Since neutron fluxes are much larger on ITER, at ex-vessel sensor locations, than those achievable on JET, the sensors have been tested in ITER-relevant radiation environment in nuclear research reactors.