In a fuel injection system of an internal combustion engine, including, but not limited to a high pressure pump adapted to deliver a fuel mass conveyed from a fuel tank to a high pressure region to which a plurality of injectors is connected, including a fuel metering device for adjusting the fuel quantity to be delivered to the high pressure region, a pressure sensor for measuring fuel pressure in the high pressure region; and an electronic control unit arranged for driving the fuel metering device through an energizing signal (Inom; Iact) as a function of the measured fuel pressure and the engine operating mode, where the electronic control unit is arranged for determining the energizing signal (Inom; Iact) for driving the fuel metering device according to a nominal characteristic curve (A) of the high pressure pump representative of a predetermined relationship between the energizing signal and the fuel mass delivered by the pump, the operating pressure is controlled by driving the fuel metering device with an energizing signal determined according to an updated characteristic curve (B) estimated by applying to the nominal characteristic curve (A) an offset between the actual energizing signal value (Iact) related to the controlled fuel mass delivered to the high pressure region in a predetermined engine operating point and a nominal energizing signal (Inom) related to the controlled fuel mass according to the nominal characteristic curve (A).