Thus relates to a movable backing system for clamping plate-shaped backing work pieces for friction agitation welding and friction stir welding and to a friction agitation or stir welding system. The movable backing system comprises an upper unit to be arranged above the backing work pieces to support the backing work pieces, a lower unit to be arranged below the backing work pieces to support the backing work pieces, and a connecting means connecting the upper and the lower unit to be arranged in a gap between the backing work pieces to be welded.