The effect of an electric field E = 120 MV m^-1 on the critical current I_c and the current-voltage characteristics of yttrium-based high-T_c ceramics have been studied experimentally with data on the electric field effect related to the critical current behaviour with temperature near the critical temperature T_c. The objects of study were ceramics with stoichiometric composition YBa₂Cu₃O_y, fabricated in different sintering conditions (123-I and 123-II samples), copper-deficient YBa₂Cu_3-xO_y (D samples) and YBa₂Cu_3-xO_y/Ag_x ceramics (S samples with silver present in amount equal to the copper deficiency 0<x0.4). In D and 123-I samples studied at 77 K, the applied electric field increases I_c and reduces substantially the resistivity at I>I_c, whereas in S and 123-II samples no field effect is observed. The results of the measurements of the temperature dependence of I_c near the critical temperature confirmed the presence of superconductor-insulator-superconductor tunnel intergrain junctions (SIS) in D and 123-I samples, and superconductor-normal metal-superconductor proximity junctions (SNS) in S and 123-II samples. The conclusion is drawn that the electric-field effect correlates with the existence of SIS-type weak links at grain boundaries in the ceramic.