A hybrid powertrain unit comprises an engine and a gearbox with a primary shaft connectable to an engine shaft via a clutch and a secondary shaft with an output pinion meshing with a first differential crown wheel. An electric machine is configured to function as an electric motor and generator, having a shaft connected by a transmission to a second differential crown wheel. The transmission, arranged between the electric machine shaft and the second crown wheel includes a first engagement device. The electric machine shaft is connected to the engine shaft by a belt transmission including a belt engaged on a first pulley connected to the electric machine shaft and a second pulley connected to the engine shaft. Between the electric machine shaft and said first pulley is a second engagement device. The engagement devices are arranged coaxially with the electric machine shaft on opposite sides of the electric machine.