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 device. 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. The unit comprises an electric machine configured to operate as an electric motor and as an electric generator, and having a shaft connected by transmission to a second crown wheel of the differential. In the transmission, arranged between the electric machine shaft of and the second crown wheel is an engagement device that can be operated via an electronically controlled actuator. The transmission that connects the electric machine shaft to said second crown wheel includes an auxiliary shift device configured for providing at least two different selectable transmission ratios.