A controller for a hybrid electric vehicle having a powertrain comprising an engine and one or more electric machines , at least one of said electric machines being operable as a propulsion motor to provide drive torque to drive the vehicle , and an energy storage means for storing electrical energy for powering at least one electric machine as a propulsion motor, the controller being arranged to control the engine and at least one electric machine to provide torque to drive the vehicle in a hybrid vehicle (HV) mode in which the engine is switched on or an electric vehicle (EV) mode in which the engine is switched off, the controller being further arranged to: monitor a value of one or more parameters associated with the vehicle ; identify a current driving style of a driver in dependence on the value of the one or more parameters; control the powertrain to operate in the HV or EV modes in dependence on the driving style of the driver; control at least one said at least one electric machine to operate as a generator to generate electrical energy for storage in the energy storage means ; control the powertrain to operate in a torque boost mode of operation in which at least one said at least one electric machine operates as a propulsion motor to provide drive torque to drive the vehicle in parallel with the engine , characterized in that the controller is further arranged to calculate a value of a driver behaviour index in dependence on the driving style of the driver and is further operable to control the engine and at least one electric machine in dependence on the value of the driver behaviour index thereby to prevent the amount of energy stored in the energy storage means from falling to a value preventing assumption of the torque boost mode; and prevent the value of the driver behaviour index changing in response to the value of longitudinal acceleration, when the value of lateral acceleration exceeds a prescribed value. monitor a value of one or more parameters associated with the vehicle ; identify a current driving style of a driver in dependence on the value of the one or more parameters; control the powertrain to operate in the HV or EV modes in dependence on the driving style of the driver; control at least one said at least one electric machine to operate as a generator to generate electrical energy for storage in the energy storage means ; control the powertrain to operate in a torque boost mode of operation in which at least one said at least one electric machine operates as a propulsion motor to provide drive torque to drive the vehicle in parallel with the engine , characterized in that the controller is further arranged to calculate a value of a driver behaviour index in dependence on the driving style of the driver and is further operable to control the engine and at least one electric machine in dependence on the value of the driver behaviour index thereby to prevent the amount of energy stored in the energy storage means from falling to a value preventing assumption of the torque boost mode; and prevent the value of the driver behaviour index changing in response to the value of longitudinal acceleration, when the value of lateral acceleration exceeds a prescribed value. <EOS>