Abstract: An injector in a spark ignition engine combustion chamber has an injector body containing a needle with a closure element co-operating with a fuel passage seat extended by a diffusion port. The needle is axially movable between a first position wherein the closure element bears against the seat and a second position wherein the closure element is away from the seat, It further has a single-piece diabolo-shaped element fixed in the downstream part of the body, The seat, the diffusion port and a bore for slidingly guiding the needle are formed in the one-piece element. It defines, with the body downstream part of the body a fuel supply chamber and has ports communicating the chamber with the bore immediately upstream of the seat and emerging tangentially in the bore.