A gas spring, having a collapse safety feature operable in the event of psure loss, is provided, for use, for example, as a counterbalance for flaps or vertically adjustable structures. A movable partition is arranged between the piston and the head of the cylinder of the gas spring, and is exposed on one side to a closed gas chamber under pressure, while on the other side the gas volume effecting the extension force of the piston rod is present between the piston and the partition. In event of pressure loss in the gas volume on either side of the piston, the movable partition will meet the piston and act as a safety to prevent abrupt release or lowering of the object for which the gas spring is to serve as a counterbalance.