A C550 pilot was dispatched without a ferry permit to a maintenance base following a diversion for a pressurization malfunction.

Narrative

In cruise flight at FL380; I noted the cabin altitude began to increase. A call was made to Center requesting a lower altitude because of a pressurization problem in the aircraft. Clearance was given to a lower altitude and at 17;000 FT the cabin pressure held below 10;000 FT or 9000 FT to be precise. The aircraft was handed over to the next Center and I requested landing at a suitable airport close by to determine what the problem could be. The decision was made to land as soon as practical to ascertain what had caused the cabin altitude pressure to increase. After landing my company officials were called and informed of the situation. I was told an aircraft was being dispatched to pickup my passengers and take them to their original destination. Management directed me to call back after the passengers were picked up and the second aircraft had departed to our destination. At this time they would give me additional instructions regarding my aircraft. After two and a half hours the second aircraft arrived and picked up my original passengers and departed for our original destination. As instructed by my superiors I telephoned and was informed to fly the aircraft back at 18;000 FT to our maintenance facility. Upon my arrival at our maintenance facility; I discovered I had flown the aircraft without the proper ferry permit issued by the FAA. I had incorrectly assumed my company had obtained the proper documents to fly the aircraft for maintenance during the two and one half hours I was on the ground. I assure you this will not happen again. Even if told by my superiors to return an aircraft; I will obtain myself the proper documentation before departure.