A burner for cooktops, comprising: - a lower burner body, which is mountable on a metal sheet of a cooktop and is configured for the application of at least two injectors (60, 62), - at least two linear Venturi ducts (42, 44) that are substantially horizontal and are provided in said lower burner body, said at least two injectors (60, 62) facing respective said at least two linear Venturi ducts (42, 44), - a first burner base with a first circular cap and provided with openings for the release of a central flame ring, and a second burner base with a second annular cap and with openings, that are provided in said second burner base and/or in said second annular cap, for the release of at least one outer annular flame ring, said first and second burner bases (4, 6) being in fluid communication with the respective two linear Venturi ducts (40, 42), characterized in that said lower burner body is made in a single piece and is provided with: - a seat for mounting a spark plug intended to interact with said first burner base, - the two linear Venturi ducts (42, 44) having substantially parallel axes, and - in a substantially central position and at the upper surface thereof, a portion having a tubular extension and projecting upward, and in that: - said first burner base is integral with said projecting tubular portion of said lower burner body, - at least one injector-holder unit that is made separately from said lower burner body and is removably constrained to said lower burner body, said injector-holder unit being provided with two portions (54, 56), which are removably insertable into corresponding holes (20, 22) obtained in said burner body in a position facing said two linear Venturi ducts (42, 44), - each of said two linear Venturi ducts (42, 44) communicates upstream with a chamber (16, 18), which carries an injector (60, 62) applied to a wall, and communicates with the exterior for sucking primary air, and communicates downstream with a chamber (40, 24) in fluid communication with one of said first and second burner bases (4, 6), - at least one of said two linear Venturi ducts (42, 44) is made separately from the lower burner body and consists of an externally cylindrical tubular element intended to be accommodated in a corresponding cylindrical seat (36, 38) obtained in the lower burner body coaxially to the circular through hole (20, 22) which accommodates the part of injector holder (54, 56), to which the respective injector (60, 62) is applied, said Venturi duct (42, 44) being insertable into the cylindrical seat (36, 38) of said lower burner body through said circular hole (20, 22) and being lockable in the desired axial position by means of locking means which can be actuated from above the lower burner body applied to the cooktop. - a lower burner body, which is mountable on a metal sheet of a cooktop and is configured for the application of at least two injectors (60, 62), - at least two linear Venturi ducts (42, 44) that are substantially horizontal and are provided in said lower burner body, said at least two injectors (60, 62) facing respective said at least two linear Venturi ducts (42, 44), - a first burner base with a first circular cap and provided with openings for the release of a central flame ring, and a second burner base with a second annular cap and with openings, that are provided in said second burner base and/or in said second annular cap, for the release of at least one outer annular flame ring, said first and second burner bases (4, 6) being in fluid communication with the respective two linear Venturi ducts (40, 42), - a seat for mounting a spark plug intended to interact with said first burner base, - the two linear Venturi ducts (42, 44) having substantially parallel axes, and - in a substantially central position and at the upper surface thereof, a portion having a tubular extension and projecting upward, - said first burner base is integral with said projecting tubular portion of said lower burner body, - at least one injector-holder unit that is made separately from said lower burner body and is removably constrained to said lower burner body, said injector-holder unit being provided with two portions (54, 56), which are removably insertable into corresponding holes (20, 22) obtained in said burner body in a position facing said two linear Venturi ducts (42, 44), - each of said two linear Venturi ducts (42, 44) communicates upstream with a chamber (16, 18), which carries an injector (60, 62) applied to a wall, and communicates with the exterior for sucking primary air, and communicates downstream with a chamber (40, 24) in fluid communication with one of said first and second burner bases (4, 6), - at least one of said two linear Venturi ducts (42, 44) is made separately from the lower burner body and consists of an externally cylindrical tubular element intended to be accommodated in a corresponding cylindrical seat (36, 38) obtained in the lower burner body coaxially to the circular through hole (20, 22) which accommodates the part of injector holder (54, 56), to which the respective injector (60, 62) is applied, said Venturi duct (42, 44) being insertable into the cylindrical seat (36, 38) of said lower burner body through said circular hole (20, 22) and being lockable in the desired axial position by means of locking means which can be actuated from above the lower burner body applied to the cooktop.