A welding or cutting unit for an automatic flexible container forming and filling machine, comprising: first and second mobile supports (1, 2) located on opposite sides of a passage through which a web of thermoplastic sheet material moves, said web having multiple overlapping layers to be welded and cut to form flexible containers in the automatic flexible container forming and filling machine; first and second thermal pressure rods (33, 34) or first and second cutting blades (3, 4) fixed to the first and second mobile supports (1, 2), respectively; and a drive mechanism converting rotary motion imparted by a motor in a back-and-forth, simultaneous and opposite movement of the first and second mobile supports (1, 2) for welding or cutting the web with said first and second thermal pressure rods (33, 34) or with said first and second cutting blades (3, 4), respectively, wherein the first and second mobile supports (1, 2), the first and second thermal pressure rods (33, 34) or first and second cutting blades (3, 4), and said drive mechanism are installed in an extractable chassis defining said passage; said extractable chassis is coupleable in a sliding manner to guide supports fixed to a structure of the automatic flexible container forming and filling machine and can be placed in a working position in said guide supports or extracted from the guide supports; and the drive mechanism has a driven coupling element installed in the extractable chassis which is automatically coupled, when the extractable chassis is placed in said working position, with a driving coupling element installed in said structure of the automatic flexible container forming and filling machine and operated by said motor. first and second mobile supports (1, 2) located on opposite sides of a passage through which a web of thermoplastic sheet material moves, said web having multiple overlapping layers to be welded and cut to form flexible containers in the automatic flexible container forming and filling machine; first and second thermal pressure rods (33, 34) or first and second cutting blades (3, 4) fixed to the first and second mobile supports (1, 2), respectively; and a drive mechanism converting rotary motion imparted by a motor in a back-and-forth, simultaneous and opposite movement of the first and second mobile supports (1, 2) for welding or cutting the web with said first and second thermal pressure rods (33, 34) or with said first and second cutting blades (3, 4), respectively, wherein the first and second mobile supports (1, 2), the first and second thermal pressure rods (33, 34) or first and second cutting blades (3, 4), and said drive mechanism are installed in an extractable chassis defining said passage; said extractable chassis is coupleable in a sliding manner to guide supports fixed to a structure of the automatic flexible container forming and filling machine and can be placed in a working position in said guide supports or extracted from the guide supports; and the drive mechanism has a driven coupling element installed in the extractable chassis which is automatically coupled, when the extractable chassis is placed in said working position, with a driving coupling element installed in said structure of the automatic flexible container forming and filling machine and operated by said motor.