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 (C) 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 (S1) from its inlet to a first intermediate position , and a decreasing area section (S2) 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 (S1) 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 (S2) 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 (C), 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 (C) 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 (C) 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 (S1) from its inlet to a first intermediate position , and a decreasing area section (S2) 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 (S1) 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 (S2) 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 (C), 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 (C) 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. <EOS>