Powertrain for a vehicle comprising: an internal combustion engine provided with a crankshaft; a first electric machine operating at least as a starter motor and located on a first side of the internal combustion engine facing in use towards a transmission of the vehicle; an accessory drive comprising: a first pulley connected to one end of the crankshaft on a second side of the engine opposite in use to the transmission of the vehicle; a second electric machine located on the second side of the internal combustion engine; a second pulley connected to the second electric machine; at least a third pulley connected to an accessory of the internal combustion engine; a belt connecting the first, the second and the third pulley (21, 22, 24) together to rotate in the same direction of rotation as the crankshaft; a tensioner acting on a span of the belt comprised between the second pulley and the third pulley; and a control unit, wherein the control unit is programmed to control the second electric machine so as to transmit an active torque to said second pulley during start-up of the internal combustion engine by means of the first electric machine, so as to maintain a predetermined minimum tension level in a span of the belt comprised between the first and the second pulleys (21, 22) in said direction of rotation. an internal combustion engine provided with a crankshaft; a first electric machine operating at least as a starter motor and located on a first side of the internal combustion engine facing in use towards a transmission of the vehicle; an accessory drive comprising: a first pulley connected to one end of the crankshaft on a second side of the engine opposite in use to the transmission of the vehicle; a second electric machine located on the second side of the internal combustion engine; a second pulley connected to the second electric machine; at least a third pulley connected to an accessory of the internal combustion engine; a belt connecting the first, the second and the third pulley (21, 22, 24) together to rotate in the same direction of rotation as the crankshaft; a tensioner acting on a span of the belt comprised between the second pulley and the third pulley; and a control unit, wherein the control unit is programmed to control the second electric machine so as to transmit an active torque to said second pulley during start-up of the internal combustion engine by means of the first electric machine, so as to maintain a predetermined minimum tension level in a span of the belt comprised between the first and the second pulleys (21, 22) in said direction of rotation. a first pulley connected to one end of the crankshaft on a second side of the engine opposite in use to the transmission of the vehicle; a second electric machine located on the second side of the internal combustion engine; a second pulley connected to the second electric machine; at least a third pulley connected to an accessory of the internal combustion engine; a belt connecting the first, the second and the third pulley (21, 22, 24) together to rotate in the same direction of rotation as the crankshaft; a tensioner acting on a span of the belt comprised between the second pulley and the third pulley; and a control unit, wherein the control unit is programmed to control the second electric machine so as to transmit an active torque to said second pulley during start-up of the internal combustion engine by means of the first electric machine, so as to maintain a predetermined minimum tension level in a span of the belt comprised between the first and the second pulleys (21, 22) in said direction of rotation.