1. A method of operating an internal combustion engine comprising at least one cylinder equipped with a pressure sensor, wherein, said engine is operated in an injector learning mode, in which an injection parameter associated with a nominal fuel quantity of a pilot injection is learned based on the following measurement routine: performing a learning pilot injection based on a candidate value for said injection parameter to influence injection of said nominal fuel quantity and determining a parameter indicative of the actually injected fuel quantity based on the in-cylinder pressure during combustion of said learning pilot injection; characterized in that said learning pilot injection is operated at an early timing in a combustion cycle such that combustion thereof is complete before the next combustion starts within the same combustion cycle; and said determination of the parameter indicative of the actually injected fuel quantity based on the in-cylinder pressure takes into account an entire combustion event of said learning pilot injection, comprising a preliminary step of measuring the in-cylinder pressure over a predetermined learning window during an engine working cycle without learning pilot injection, in order to determine a starting baseline pressure; and the determination of the parameter indicative of the injected fuel quantity for an actual fuel injection takes into account said starting baseline pressure and correcting a subsequent injection based on the parameter indicative of the actually injected fuel quantity.