A fuel metering unit including a movable element including at least one fuel passage section opening upstream towards a fuel supply conduit and opening downstream towards a conduit of use through a metering slot with a flared profile having a narrow passage section flaring as far as a wide passage section, the movable element being able to be moved with respect to a fixed element between a low flow rate position in which the metering slot is for a large part obstructed and a high flow rate position in which the metering slot is for a large part exposed, the metering slot being made in the fixed element or in the movable element and its obstruction being obtained by covering the slot with a wall of the movable element or of the fixed element.