A controller for a hybrid electric vehicle having a powertrain, the powertrain including an engine, electric propulsion means powered by energy storage means and electric generator means operable to be driven by the engine to recharge the energy storage means, the controller being operable to: receive a signal indicative of a required hybrid operating mode of the vehicle the hybrid operating modes including a first hybrid operating mode favouring prolonged operation in an electric vehicle powertrain mode by promoting charging of the energy storage means and a second hybrid operating mode favouring a reduction in fuel consumption, wherein in each hybrid operating mode, the powertrain is configured to operate in a plurality of different powertrain modes; receive a signal indicative of a state of charge of the energy storage means; determine which of the plurality of powertrain modes is appropriate for vehicle operation at a given moment, the powertrain modes including an engine charging powertrain mode in which the engine drives the generator means to recharge the energy storage means and the electric vehicle powertrain mode in which the engine is switched off and the electric propulsion means is operable to develop drive torque to drive the vehicle; and cause the powertrain to assume the appropriate powertrain mode, wherein the controller is operable to determine which of the plurality of powertrain modes is appropriate for vehicle operation in dependence at least in part on the signal indicative of the instant state of charge of the energy storage means and a reference value of state of charge, the reference value in the first hybrid operating mode being higher than that in the second hybrid operating mode, and the controller being operable to set the reference value of state of charge to one of a plurality of different respective values in dependence on the signal indicative of the required hybrid operating mode; wherein the controller is operable to determine which of the powertrain modes is appropriate at a given moment in time according to a value of a cost function for each powertrain mode, the value of the cost function being determined at least in part by reference to the signal indicative of the instant state of charge and the reference value of state of charge of the respective powertrain modes. receive a signal indicative of a required hybrid operating mode of the vehicle the hybrid operating modes including a first hybrid operating mode favouring prolonged operation in an electric vehicle powertrain mode by promoting charging of the energy storage means and a second hybrid operating mode favouring a reduction in fuel consumption, wherein in each hybrid operating mode, the powertrain is configured to operate in a plurality of different powertrain modes; receive a signal indicative of a state of charge of the energy storage means; determine which of the plurality of powertrain modes is appropriate for vehicle operation at a given moment, the powertrain modes including an engine charging powertrain mode in which the engine drives the generator means to recharge the energy storage means and the electric vehicle powertrain mode in which the engine is switched off and the electric propulsion means is operable to develop drive torque to drive the vehicle; and cause the powertrain to assume the appropriate powertrain mode, wherein the controller is operable to determine which of the plurality of powertrain modes is appropriate for vehicle operation in dependence at least in part on the signal indicative of the instant state of charge of the energy storage means and a reference value of state of charge, the reference value in the first hybrid operating mode being higher than that in the second hybrid operating mode, and the controller being operable to set the reference value of state of charge to one of a plurality of different respective values in dependence on the signal indicative of the required hybrid operating mode; wherein the controller is operable to determine which of the powertrain modes is appropriate at a given moment in time according to a value of a cost function for each powertrain mode, the value of the cost function being determined at least in part by reference to the signal indicative of the instant state of charge and the reference value of state of charge of the respective powertrain modes.