A method of reducing interference in a first wireless communications system, the first wireless communications system being operable in use to take part in a spectrum assignment process involving at least a second wireless communications system, the method comprising defining an exclusion zone in proximity to the first wireless communications system, the exclusion zone indicating that any mobile communications apparatus located within the exclusion zone is not permitted to communicate with the second wireless communications system using a portion of spectrum which was pre-assigned to the first wireless communications system and which is currently re-assigned to the second wireless communications system as part of the spectrum assignment process; and varying the local size of a part only of the exclusion zone in dependence on a level of interference inflicted on the first wireless communications system.