Abstract: High turn down ratio burner, comprising a burner housing (1) having a connection (2) for a combustible gas/air mixture and a substantially tubular mixture passage (3) leading to a constricted burner throat or mouth (4), via which passage the burner is connectable to a combustion chamber (7) which is part of a heat exchanger (6) or forms a flue passage leading to such heat exchanger, further comprising a flame arrester (8), which is a flashback preventer, in the form of a porous, gas/air mixture-permeable hollow body covering the entire flow-through area towards the burner throat (4) and arranged with its concave side towards the burner throat (4), at a short distance upstream thereof, a number of constricted auxiliary gas passages (9) being arranged around the burner throat.