Method for premixing air with a gaseous fuel for being burned in a combustion chamber, said method comprising the steps of: guiding said air in an air stream along a burner axis through a coaxial air tube into a combustion chamber arranged at an end of said air tube; impressing a swirl on said air stream by passing it through a first swirl device concentrically arranged within said air tube and comprising a plurality of radially oriented first blades; injecting gaseous fuel into said air stream at said first swirl device; and mixing said air in said air stream with the injected gaseous fuel in a first mixing zone arranged just after said first swirl device; sending the mixed fuel/air stream leaving said first mixing zone through at least one second swirl device concentrically arranged within said air tube and comprising a plurality of radially oriented second blades to reduce the swirl of the mixed fuel/air stream; injecting gaseous fuel into said mixed fuel/air stream at said second swirl device; further mixing said mixed fuel/air stream with the injected gaseous fuel in a second mixing zone arranged just between said second swirl device and said combustion chamber; characterized in that said first swirl device has a first swirl number, said second swirl device has a second swirl number, and said second swirl number is smaller than said first swirl number and wherein the cylindrical cross section of the blades (29, 30) of the first and second swirl device (14, 16) has the shape of an airfoil in order to reduce a pressure drop, each of the blades (29, 30) of the first and second swirl device (14, 16) having a leading edge and a trailing edge and the leading edge of the blades of the second swirl device being aligned in terms of inflow angle with outflow angle of the trailing edge of the blades of the first swirl device. guiding said air in an air stream along a burner axis through a coaxial air tube into a combustion chamber arranged at an end of said air tube; impressing a swirl on said air stream by passing it through a first swirl device concentrically arranged within said air tube and comprising a plurality of radially oriented first blades; injecting gaseous fuel into said air stream at said first swirl device; and mixing said air in said air stream with the injected gaseous fuel in a first mixing zone arranged just after said first swirl device; sending the mixed fuel/air stream leaving said first mixing zone through at least one second swirl device concentrically arranged within said air tube and comprising a plurality of radially oriented second blades to reduce the swirl of the mixed fuel/air stream; injecting gaseous fuel into said mixed fuel/air stream at said second swirl device; further mixing said mixed fuel/air stream with the injected gaseous fuel in a second mixing zone arranged just between said second swirl device and said combustion chamber; characterized in that said first swirl device has a first swirl number, said second swirl device has a second swirl number, and said second swirl number is smaller than said first swirl number and wherein the cylindrical cross section of the blades (29, 30) of the first and second swirl device (14, 16) has the shape of an airfoil in order to reduce a pressure drop, each of the blades (29, 30) of the first and second swirl device (14, 16) having a leading edge and a trailing edge and the leading edge of the blades of the second swirl device being aligned in terms of inflow angle with outflow angle of the trailing edge of the blades of the first swirl device.