Fuel injector comprising a body in which a needle moves between an open position and a closed position under the influence of the pressure of fuel in a control chamber, a chamber into which there opens a high pressure feed channel (146, 156, 130) and from which there departs in the direction of a low pressure return circuit a discharge channel (158, 162, 164) controlled by a two-way solenoid valve so that when the solenoid valve is in the closed position the pressure in the control chamber increases pushing the needle towards the closed position and when the solenoid valve is in the open position fuel is discharged from the control chamber, the pressure in which falls, allowing the needle to move into the open position, in which the injector further comprises means (26, 114) for reducing dynamic leaks by preventing the fuel from flowing directly from the feed channel to the return circuit when the solenoid valve is in the open position, said means (26, 114) for reducing dynamic leaks comprising a moving member which moves between an open position and a closed position under the influence of the pressure difference in the fuel within the control chamber and a second control chamber in fluid communication with the discharge channel, and in which the moving member is a cylindrical piston provided on its lateral cylindrical surface with an annular groove, the cylindrical piston being arranged to slide within a bore of the injector body, the feed channel (148, 156) opening into the annular space defined by the annular groove in said bore, said piston moving into an open position when the solenoid valve is in the closed position so that high pressure fuel can enter the control chamber and moving into a closed position when the solenoid valve is in an open position so as to prevent high pressure fuel from entering the control chamber, and in which said bore is coaxial with the bore in which the needle slides, and in which the piston comprises a single cylindrical body extending axially between an undersurface facing the needle, said undersurface forming the top of the control chamber and an upper surface opposite the undersurface, said upper surface being in contact with a transverse surface of the injector body when the piston has moved into the closed position, the fuel injector further comprising a spring located in the control chamber and compressed between a shoulder of one piece with the needle and the undersurface of the piston so as to press the needle towards its closed position and the piston towards its closed position at all times, and in which the piston is provided with a first channel extending from the outer wall of the piston to the upper surface and also with a second channel extending between the undersurface and the upper surface, the piston being located within its bore so that the feed channel (148, 156) is in permanent fluid communication with the first channel, and in which the piston is further provided on its upper surface with a protuberance forming a leaktight barrier when the piston is in the closed position, said barrier being then in leaktight contact with said transverse surface of the injector body, the first channel and the second channel opening onto the upper surface of the piston on either side of said barrier, said second control chamber being the space located between the transverse surface of the injector body and the upper surface of the piston and delimited by the side of the barrier where the second channel and the discharge conduit open.