A modulating gas burner which is designed for operation with premixing of a combustible air and gas mixture, comprising: - a burner body mounted on a shutter element for sealing a combustion chamber of the burner, in the region of a through aperture in the shutter through which the combustible mixture is supplied to the burner, - a burner head to which the combustible air and gas mixture, which is introduced into the combustion chamber through the aperture, can be supplied, - a device for guiding the flow of the mixture, which device imparts rotational components to the velocity of the flow to the burner head, - a flash-back prevention device positioned in the burner head, the power at the burner being modulated within a predetermined modulation range between a minimum power condition in which the burner operates predominantly in the manner of a premix burner without flame detachment or flash-back, and a maximum power condition in which the burner operates predominantly in the manner of a burner with recirculation of the flows of the reagents of the combustible mixture, the flow guide device being structurally independent of the flash-back prevention device, wherein the flow guide device is produced integrally with the shutter element, in the region of the through aperture, wherein the flow guide device comprises a plurality of radial deflector fins which are integral with the shutter element and are formed inside the through aperture of the shutter element, the fins being spaced apart circumferentially and inclined to the axial direction of the flow through the head so as to impart a rotational component to the flow that is conveyed through respective passageway sections defined between each pair of adjacent fins, and wherein the burner body has a tubular portion with a cylindrical wall which carries, at one of its axial ends, a flange for connection to the shutter element, and is extended at the opposite axial end by a portion with a tapered wall, the body being fixed to the shutter element by means of the flange with the interposition of the flash-back prevention device. - a burner body mounted on a shutter element for sealing a combustion chamber of the burner, in the region of a through aperture in the shutter through which the combustible mixture is supplied to the burner, - a burner head to which the combustible air and gas mixture, which is introduced into the combustion chamber through the aperture, can be supplied, - a device for guiding the flow of the mixture, which device imparts rotational components to the velocity of the flow to the burner head, - a flash-back prevention device positioned in the burner head, the power at the burner being modulated within a predetermined modulation range between a minimum power condition in which the burner operates predominantly in the manner of a premix burner without flame detachment or flash-back, and a maximum power condition in which the burner operates predominantly in the manner of a burner with recirculation of the flows of the reagents of the combustible mixture, the flow guide device being structurally independent of the flash-back prevention device, wherein the flow guide device is produced integrally with the shutter element, in the region of the through aperture, wherein the flow guide device comprises a plurality of radial deflector fins which are integral with the shutter element and are formed inside the through aperture of the shutter element, the fins being spaced apart circumferentially and inclined to the axial direction of the flow through the head so as to impart a rotational component to the flow that is conveyed through respective passageway sections defined between each pair of adjacent fins, and wherein the burner body has a tubular portion with a cylindrical wall which carries, at one of its axial ends, a flange for connection to the shutter element, and is extended at the opposite axial end by a portion with a tapered wall, the body being fixed to the shutter element by means of the flange with the interposition of the flash-back prevention device.