The system comprises control devices (CS) arranged to implement a stop-start function in which, during a stop stage, the internal combustion engine (1) is disconnected from the electric machine (4) and from the transmission system (2) and is switched off in first predetermined operating conditions of the vehicle (V) and then, in a subsequent start stage, is automatically restarted by an electric machine (4), operating as a motor, and is then reconnected to the transmission system (2).