An improved gas burner (2, 86) comprising: - an injector holder (4, 88) provided with means (20, 22) suitable for its installation in the upper sheet metal of a cooking hob, and comprising a cup-shaped central cavity, to the base of which a central injector is applied, and further comprising two lateral cup-shaped cavities (12, 12'), to the base of which two lateral injectors (16, 18) are applied, - at least one gas inlet provided in said injector holder (4, 88) and connected to said injectors (14, 16, 18), - a flame divider resting on the injector holder (4, 88) and defining a central distribution chamber provided in its substantially cylindrical lateral wall with apertures for generating a central ring of flames, and an annular distribution chamber which is separated from said central distribution chamber by an annular cavity and is provided in its outer lateral wall with apertures for generating a ring of outwardly directed flames, - a circular closure cover for said central distribution chamber and an annular closure cover for said annular distribution chamber, - at least one connection passage between the space surrounding the burner and said annular cavity of the flame divider, - wherein the annular chamber of the flame divider incorporates two lateral conduits which: - are disposed with their axis substantially vertical, - have their lower ends facing said lateral injectors (16, 18), and - are configured such as to generate an essentially vertical venturi effect, and such that the gas and air flow emerging therefrom and entering the annular distribution chamber is substantially already completely mixed, characterised in that - said cup-shaped cavities (10, 12, 12') are closed both on their base and laterally while they are opened upwards, - said lower ends of the two lateral conduits are disposed slightly higher level than the upper edge of the injector holder (4, 88), and - said two lateral conduits present an upper end portion which projects and emerges from the base of the annular distribution chamber. - an injector holder (4, 88) provided with means (20, 22) suitable for its installation in the upper sheet metal of a cooking hob, and comprising a cup-shaped central cavity, to the base of which a central injector is applied, and further comprising two lateral cup-shaped cavities (12, 12'), to the base of which two lateral injectors (16, 18) are applied, - at least one gas inlet provided in said injector holder (4, 88) and connected to said injectors (14, 16, 18), - a flame divider resting on the injector holder (4, 88) and defining a central distribution chamber provided in its substantially cylindrical lateral wall with apertures for generating a central ring of flames, and an annular distribution chamber which is separated from said central distribution chamber by an annular cavity and is provided in its outer lateral wall with apertures for generating a ring of outwardly directed flames, - a circular closure cover for said central distribution chamber and an annular closure cover for said annular distribution chamber, - at least one connection passage between the space surrounding the burner and said annular cavity of the flame divider, - wherein the annular chamber of the flame divider incorporates two lateral conduits which: - are disposed with their axis substantially vertical, - have their lower ends facing said lateral injectors (16, 18), and - are configured such as to generate an essentially vertical venturi effect, and such that the gas and air flow emerging therefrom and entering the annular distribution chamber is substantially already completely mixed, characterised in that - said cup-shaped cavities (10, 12, 12') are closed both on their base and laterally while they are opened upwards, - said lower ends of the two lateral conduits are disposed slightly higher level than the upper edge of the injector holder (4, 88), and - said two lateral conduits present an upper end portion which projects and emerges from the base of the annular distribution chamber. - said cup-shaped cavities (10, 12, 12') are closed both on their base and laterally while they are opened upwards, - said lower ends of the two lateral conduits are disposed slightly higher level than the upper edge of the injector holder (4, 88), and - said two lateral conduits present an upper end portion which projects and emerges from the base of the annular distribution chamber.