The present invention relates to a fuel injection system for an internal combustion engine with a fuel injector, said injector comprising an inlet port (2); a spill port (3); a pressure intensifier comprised of a piston (6) forming a working chamber (10) and a plunger (7) forming a compression chamber (12), wherein the working chamber (10) is adapted to be connected either to the inlet port (2) or to the spill port (3) according to the commands from an engine management system in order to enable the pressure intensifier (6, 7) to perform injections; a nozzle (14) with a needle (15), a locking chamber (17), a means (16) biasing the needle to close the nozzle and an outlet chamber (18) connected to the compression chamber (12); a non-return valve (19), the inlet of the non-return valve (19) being connected to the inlet port (2) and the outlet of the non-return valve (19) being connected to the compression chamber (12); a cut-off channel (20) connected to the nozzle locking chamber (17); a control channel (22); an additional control valve (23) installed between the control channel (22) and the spill port (3); a link channel (24) connecting the control channel (22) to the inlet port (2). The invention is characterized in that the flow areas of the link channel (24) and the additional control valve (23) are such that when the additional control valve (23) is open the pressure in the control channel (22) becomes less than the pressure upstream of the link channel (24); said plunger (7) being adapted so as to connect the cut-off channel (20) to the compression chamber (12) at an injection cut-off position of the plunger (7) and adapted to connect the cut-off channel (20) to the control channel (22) at other positions of the plunger (7). By means of the invention, an improved fuel injector having an increased range of electronic control of an injection curve shape, improved stability of fuel delivery and a simplified design is accomplished.