Nozzle assembly of a dual fuel injector of an internal combustion engine, said nozzle assembly extending along a main axis from a large upper end adapted to cooperate with an actuation member of the injector to a narrow tip end, said nozzle assembly comprising coaxially arranged along said main axis an elongated body having a peripheral wall surrounding an inner space which portion proximal the upper end defines an upper guiding bore for axially guiding an outer valve member arranged therein, said outer valve member being itself provided with a axial bore in which is axially guided an inner valve member; and wherein the nozzle assembly further comprises - a first fuel F1 conduit comprising said axial bore and extending from an inlet arranged in the large upper end face of the body to a first set of injection holes arranged in the narrow tip end of the outer valve member and, - a second fuel F2 conduit comprising said inner space and extending from another inlet arranged in the large upper end face of the body to a second set of injection holes arranged in the narrow tip end of the body, so that in use, a first fuel, such as pressurized diesel or gasoline, flows inside the outer valve member and, a second fuel, such as a compressed gas fills and flows in the inner space surrounding said outer valve member, wherein the nozzle assembly is further provided with an anti-leakage feature comprising a cylindrical tubular sleeve axially extending in said inner space from an upper end attached to the body (28, 30) to a distant lower end, the outer valve member being axially guided through said sleeve and, wherein the sleeve is arranged as an extension of the upper guiding bore and wherein a clearance is defined between the outer valve member and the bore, or the sleeve and, wherein the inner diameter of the sleeve is smaller than or identical to the inner diameter of the upper guiding bore and, wherein said clearance is not constant, the sleeve or the outer valve member being provided on at least part of the length with a taper. and wherein the nozzle assembly further comprises - a first fuel F1 conduit comprising said axial bore and extending from an inlet arranged in the large upper end face of the body to a first set of injection holes arranged in the narrow tip end of the outer valve member and, - a second fuel F2 conduit comprising said inner space and extending from another inlet arranged in the large upper end face of the body to a second set of injection holes arranged in the narrow tip end of the body, - a first fuel F1 conduit comprising said axial bore and extending from an inlet arranged in the large upper end face of the body to a first set of injection holes arranged in the narrow tip end of the outer valve member and, - a second fuel F2 conduit comprising said inner space and extending from another inlet arranged in the large upper end face of the body to a second set of injection holes arranged in the narrow tip end of the body, so that in use, a first fuel, such as pressurized diesel or gasoline, flows inside the outer valve member and, a second fuel, such as a compressed gas fills and flows in the inner space surrounding said outer valve member, wherein the nozzle assembly is further provided with an anti-leakage feature comprising a cylindrical tubular sleeve axially extending in said inner space from an upper end attached to the body (28, 30) to a distant lower end, the outer valve member being axially guided through said sleeve and, wherein the sleeve is arranged as an extension of the upper guiding bore and wherein a clearance is defined between the outer valve member and the bore, or the sleeve and, wherein the inner diameter of the sleeve is smaller than or identical to the inner diameter of the upper guiding bore and, wherein said clearance is not constant, the sleeve or the outer valve member being provided on at least part of the length with a taper.