A vacuum brake booster (10) is described, having a vacuum chamber (16) and a working chamber (18) separated from one another by a movable wall (14), and a control valve (20), which comprises a housing (12) coupled workingly to the movable wall (14) and which to achieve a pressure difference at the movable wall (14) is capable of controlling the supply of atmospheric pressure or above-atmospheric pressure to the working chamber (18) in dependence upon the displacement of an actuating piston (28). The vacuum brake booster (10) has an emergency braking aid comprising a permanent magnet (38) and an armature (36), which cooperates with the permanent magnet (38) and is resiliently biased counter to actuating direction and during an emergency braking operation is drawn into abutment with the permanent magnet (38), with the result that the control valve (20) is held open for the supply of atmospheric pressure to the working chamber (18). The vacuum brake booster (10) comprises a coupling device (90) for detachably coupling the armature (36) to the actuating piston (28).