An electromagnetic fuel injector having a cylindrical symmetry around a longitudinal axis and comprising: an injection jet; an injection valve, which is provided with a mobile plunger, so as to adjust the flow of fuel through the injection jet; an electromagnetic actuator to move the plunger between a closed position and an open position of the injection valve and provided with at least one first electromagnet comprising a first coil, a first fixed armature, and a first mobile armature, which is mechanically connected to the plunger; a closing spring, which pushes the plunger towards the closed position; a supporting body, which has a tubular shape and is provided with a central channel, which houses the first fixed armature and the first mobile armature; and an extension, which is partially arranged on the inside of the supporting body, is arranged along the longitudinal axis on the first fixed armature, and is mechanically constrained to the supporting body by means of a first annular weld; wherein the first fixed armature is mechanically locked in a fixed position on the inside of the supporting body by means of a second annular weld, which constrains the first fixed armature to the extension; the injector is characterized in that: the first coil is fitted on the outside relative to the supporting body; the first fixed armature is arranged along the longitudinal axis between the extension and the first mobile armature; and the first fixed armature has no direct mechanical constraint to the supporting body and is kept still on the inside of the supporting body only through the second annular weld, which mechanically constrains the first fixed armature to the extension. an injection jet; an injection valve, which is provided with a mobile plunger, so as to adjust the flow of fuel through the injection jet; an electromagnetic actuator to move the plunger between a closed position and an open position of the injection valve and provided with at least one first electromagnet comprising a first coil, a first fixed armature, and a first mobile armature, which is mechanically connected to the plunger; a closing spring, which pushes the plunger towards the closed position; a supporting body, which has a tubular shape and is provided with a central channel, which houses the first fixed armature and the first mobile armature; and an extension, which is partially arranged on the inside of the supporting body, is arranged along the longitudinal axis on the first fixed armature, and is mechanically constrained to the supporting body by means of a first annular weld; wherein the first fixed armature is mechanically locked in a fixed position on the inside of the supporting body by means of a second annular weld, which constrains the first fixed armature to the extension; the injector is characterized in that: the first coil is fitted on the outside relative to the supporting body; the first fixed armature is arranged along the longitudinal axis between the extension and the first mobile armature; and the first fixed armature has no direct mechanical constraint to the supporting body and is kept still on the inside of the supporting body only through the second annular weld, which mechanically constrains the first fixed armature to the extension. the first coil is fitted on the outside relative to the supporting body; the first fixed armature is arranged along the longitudinal axis between the extension and the first mobile armature; and the first fixed armature has no direct mechanical constraint to the supporting body and is kept still on the inside of the supporting body only through the second annular weld, which mechanically constrains the first fixed armature to the extension.