A closure for holding closed a door which is able to hinge in a frame, having a securing device which can be positioned on a first edge of the closed door and also a tightening unit which can be positioned on a second edge of the closed door and a cable or strap which, when the door is closed, runs through a gap between the door and frame and is fastened at one end to the securing device and fastened at the other end in the tightening unit, the tightening unit having inter alia a base plate with an opening for feeding through the cable or strap and also a tightening mechanism for tensioning and holding under tension the cable or strap guided through the opening and a cap with a lock which can be positioned on the base plate so as to produce a seal around the tightening mechanism. The securing device has a support extending on both sides of the cable or strap over a distance which is greater than the gap between the closed door and the frame, which distance is preferably greater than 5 mm.