A premixing burner is disclosed for operating a combustion chamber with a liquid and/or gaseous fuel, with a swirl generator for a combustion inflow air stream for forming a swirl flow, and with injection of fuel into the swirl flow. The swirl generator is adjacent to the combustion chamber indirectly via a mixing zone or directly, in each case via a burner outlet, a cross-sectional widening at the burner outlet being provided which, is discontinuous in the flow direction of the swirl flow and through which the swirl flow bursts open so as to form a backflow zone. A contour locally narrowing the flow cross section of the swirl generator or of the mixing zone in the flow direction can be provided upstream of the burner outlet.