A hybrid powertrain unit comprises an engine, and a gearbox device with a primary shaft connectable to a shaft of the engine via a clutch. The gearbox device comprises a secondary shaft with an output pinion meshing with a first crown wheel of a differential, the casing of which is rigidly connected to a casing of the gearbox device. An electric machine is configured to function as an electric motor and an electric generator, having a shaft connected by a transmission to a second crown wheel of the differential. In the transmission, arranged between the electric machine shaft and the second crown wheel is an engagement device that can be actuator driven. The electric machine shaft is connected to the engine shaft, on a side opposite to the gearbox device by a belt transmission that controls vehicle auxiliary devices, which are mounted coaxially with respect to the electric machine.