A furnace having a wall and a low NO x emission industrial burner wherein said burners includes: - a hollow metal body defining a longitudinal axis, - a refractory diffuser, arranged at an open end of the hollow body so as to define a volume within said hollow body, - a first combustion area, located in a cavity of said refractory diffuser and adapted to be in communication with a second combustion area (27, 28) downstream of the first combustion area, - a first fuel gas injection lance, centrally arranged within said hollow body, in communication at one open end with said first combustion area for injecting fuel gas therein, - at least one second fuel gas injection lance, laterally arranged within said hollow body, adapted to be directly in communication at one open end with said second combustion area (27, 28) for directly injecting fuel gas therein, - switching means adapted to switch the injection of the fuel gas from the first fuel gas injection lance to said at least one second fuel gas injection lance or vice versa, for performing transition from a multistage flame combustion operating mode to a flameless combustion operating mode, - a conduit for feeding comburent air into said volume, - first longitudinal holes provided in the refractory diffuser and arranged along a first cylindrical side surface, the outlet section of which is directly in communication with the first combustion area for injecting a first portion of comburent air directly in the first combustion area, - second longitudinal holes provided in the refractory diffuser and arranged along a second cylindrical side surface, which is coaxial and external to the first surface, the outlet section of which is adapted to be directly in communication with the second combustion area (27, 28) for injecting a second portion of comburent air directly in the second combustion area (27, 28). wherein said burner is adapted to perform the multistage flame combustion operating mode during the heating step of the combustion chamber until the autoignition temperature of the fuel gas is reached, and indifferently, the multistage flame combustion operating mode or the flameless combustion operating mode at temperatures higher than said autoignition temperature, thus ensuring low NO x emissions at any operating conditions, characterized in that the second combustion area (27, 28) is located in the part of said wall in which the burner is adapted to be inserted, and in that said at least one second fuel gas injection lance is at least partially housed within a respective hole of said second longitudinal holes, and the ratio between a diameter of the at least one second fuel gas injection lance and a diameter of said respective hole is in the range between 0,1 and 1. - a hollow metal body defining a longitudinal axis, - a refractory diffuser, arranged at an open end of the hollow body so as to define a volume within said hollow body, - a first combustion area, located in a cavity of said refractory diffuser and adapted to be in communication with a second combustion area (27, 28) downstream of the first combustion area, - a first fuel gas injection lance, centrally arranged within said hollow body, in communication at one open end with said first combustion area for injecting fuel gas therein, - at least one second fuel gas injection lance, laterally arranged within said hollow body, adapted to be directly in communication at one open end with said second combustion area (27, 28) for directly injecting fuel gas therein, - switching means adapted to switch the injection of the fuel gas from the first fuel gas injection lance to said at least one second fuel gas injection lance or vice versa, for performing transition from a multistage flame combustion operating mode to a flameless combustion operating mode, - a conduit for feeding comburent air into said volume, - first longitudinal holes provided in the refractory diffuser and arranged along a first cylindrical side surface, the outlet section of which is directly in communication with the first combustion area for injecting a first portion of comburent air directly in the first combustion area, - second longitudinal holes provided in the refractory diffuser and arranged along a second cylindrical side surface, which is coaxial and external to the first surface, the outlet section of which is adapted to be directly in communication with the second combustion area (27, 28) for injecting a second portion of comburent air directly in the second combustion area (27, 28). wherein said burner is adapted to perform the multistage flame combustion operating mode during the heating step of the combustion chamber until the autoignition temperature of the fuel gas is reached, and indifferently, the multistage flame combustion operating mode or the flameless combustion operating mode at temperatures higher than said autoignition temperature, thus ensuring low NO x emissions at any operating conditions,