Abstract: A burner (1), fed with a mixture of fuel and air, comprising a body (2) consisting of two half-shells having a plane of symmetry, inside said body a Venturi tube (3) being defined, provided with and inlet (4) into which said mixture of fuel and air is introduced, said Venturi tube (3) communicating upwardly with a distributing chamber (5) of said mixture of fuel and air, said distributing chamber (5) communicating upwardly with a burner head (8), closed upwardly by a diffuser (9), on which openings are made for the outflow and the subsequent combustion of said mixture of fuel and air, said distributing chamber (5) exhibits a first portion (6) and a second portion (7), said first portion (6), communicating with said Venturi tube, comprising at least a first stretch (6a; 6c; 6e) having a cross section of substantially constant width and at least a second stretch (6b; 6d) having cross section that increases progressively along a direction parallel to said plane of symmetry towards an end of said distributing cha