A direct injection fuel system for injecting hydrogen fuel into a gas turbine combustor, the fuel injection system comprising a plurality of fuel injector blocks, each of which includes: a fuel admission duct having an inlet for receiving hydrogen fuel from a fuel supply, an outlet for delivering hydrogen fuel into the combustor and a central axis extending from said inlet to said outlet; an air admission duct located around the periphery of the fuel admission duct, having an inlet for receiving air from a diffuser and an outlet for delivering air into the combustor for mixing with the hydrogen fuel; wherein the air admission duct is concentric with the fuel admission duct; wherein the air admission duct comprises a constant area section from its inlet to a first intermediate position, and a decreasing area section between the first intermediate position and the outlet; wherein the fuel admission duct is defined by a peripheral wall extending from its inlet to its outlet; wherein the constant area section of the air admission duct is defined by a straight portion of the peripheral wall and a parallel, straight portion of the wall of air admission duct; wherein the decreasing area section is defined by a combination of the straight portion of the peripheral wall and an inclined portion of the wall of air admission duct, wherein the inclined portion of the wall of air admission duct is inclined inward at an angle α relative to the central axis, wherein the peripheral wall comprises a chamfered section between a second intermediate position downstream of the first intermediate position, and its outlet, wherein the peripheral wall is chamfered in towards the central axis with an angle β, wherein the angles α and β are each between 15 and 20 degrees, and wherein the angles α and β are equal. a fuel admission duct having an inlet for receiving hydrogen fuel from a fuel supply, an outlet for delivering hydrogen fuel into the combustor and a central axis extending from said inlet to said outlet; an air admission duct located around the periphery of the fuel admission duct, having an inlet for receiving air from a diffuser and an outlet for delivering air into the combustor for mixing with the hydrogen fuel; wherein the air admission duct is concentric with the fuel admission duct; wherein the air admission duct comprises a constant area section from its inlet to a first intermediate position, and a decreasing area section between the first intermediate position and the outlet; wherein the fuel admission duct is defined by a peripheral wall extending from its inlet to its outlet; wherein the constant area section of the air admission duct is defined by a straight portion of the peripheral wall and a parallel, straight portion of the wall of air admission duct; wherein the decreasing area section is defined by a combination of the straight portion of the peripheral wall and an inclined portion of the wall of air admission duct, wherein the inclined portion of the wall of air admission duct is inclined inward at an angle α relative to the central axis, wherein the peripheral wall comprises a chamfered section between a second intermediate position downstream of the first intermediate position, and its outlet, wherein the peripheral wall is chamfered in towards the central axis with an angle β, wherein the angles α and β are each between 15 and 20 degrees, and wherein the angles α and β are equal.