Abstract: An improved burner (4) for fuel oil and fuel gas with low NOx production, wherein the combustion air is subdivided in three streams and with flow rate control swirlers in each air streams. The stream of primary air is intercepted by a swirler comprising an array of radially fixed tabs (15) mounted in correspondence to the outlet section in the combustion chamber (3), while the secondary and tertiary air swirlers comprise tabs (18, 19) turnable around axes disposed perpendicular and parallel to the central axis of the burner, respectively. The primary and secondary air inlets have a variable, continuously adjustable section. Furthermore, there are provided mass rate meters (24, 25) near the outlet section of the secondary and tertiary air streams in the combustion chamber (3) substantially unaffected by the vorticity induced by said swirlers.