The transmission (10) comprises a main gearbox (12) made as a mechanical gearbox with a plurality of gears and an electric machine (14). The main gearbox (12) comprises at least one primary shaft (24), at least one secondary shaft (26), and a plurality of gear sets (30, 44, 32, 46, 34, 48, 36, 50, 38, 52, 40, 54) adapted to implement each a respective gear (I, II, III, IV, V, VI) and comprising each a driving gearwheel (30, 32, 34, 36, 38, 40) carried by a respective primary shaft (24) and a driven gearwheel (44, 46, 48, 50, 52, 54) carried by a respective secondary shaft (26) and directly or indirectly meshing with the respective driving gearwheel (30, 32, 34, 36, 38, 40). A first gearwheel (30) and a second gearwheel (44) meshing with each other are idly mounted on the primary shaft (24) and on the secondary shaft (26), respectively, and are selectively connectable for rotation with the respective shaft (24, 26) by means of a first coupling device (42) and of a second coupling device (56), respectively. The electric machine (14) is releasably connected to the first gearwheel (30) or to the second gearwheel (44), in such a manner that with the first coupling device (42) in an engaged position the electric machine (14) is able to transmit torque to the primary shaft (24) on which the first gearwheel (30) is mounted, while with the second coupling device (56) in an engaged position the electric machine (14) is able to transmit torque to the secondary shaft (26) on which the second gearwheel (44) is mounted. The transmission (10) further comprises a secondary gearbox (16) with two or more gears interposed between the electric machine (14) and the first (30) or the second gearwheel (44).