Sabotage operations were ordered from Berlin in the late autumn of 1940 , but actual work did not start until early 1941 . The first operations were unsuccessful . A first attempt to smuggle a bomb into Gibraltar was aborted , as the timing device was faulty . In February there was a large explosion in the North Tunnel , and in April a bomb blew up near the airfield . In June 1941 , however , the British intelligence foiled a new attempt , by a German agent , to attach a mine alongside an Allied cargo ship . Another attempt failed when Plazas placed a bomb inside an ammunition store but was not able to bring the explosive . It was not until 1942 that the operations begun to succeed . In January 1942 , two Spanish agents manage to destroy two aircraft at the North Front landing strip .