1. A method of operating an electronic flight planning system for aircraft, the method comprising: receiving as data input to the system from one or more users a plurality of flight plans forming a plurality of received flight plans, each received flight plan defining a flight by one of a plurality of aircraft; determining, for each flight by one of said plurality of aircraft, as defined in the respective received flight plan, a three-dimensional region of potential conflict, representative both of an uncertainty in the position of the aircraft and of a region of air exclusion appropriate for the aircraft or for the respective received flight plan; determining, on the basis of the determined regions of potential conflict, whether any one of the received flight plans is in conflict with any one of the other received flight plans; and before the flight of one of said plurality of aircraft, communicating the one or more conflicts to the users associated with said one or more conflicts.