Whilst in cruise at FL250 the aircraft de-pressurised and the oxygen masks deployed. The crew declared a PAN, executed an emergency descent, and performed the Rapid Depressurisation and Emergency Descent drills. During the descent it was determined that the selector switch for the cabin pressure controller was in the DITCH position which had caused all cabin pressure to be lost. The crew re-selected the switch to its normal position and regained control of the pressurisation. The descent was terminated at FL160 and the flight continued to its destination. It was determined that the incident occurred when the co-pilot's shoulder contacted the switch while he was returning to his seat. On this and one other aircraft in the operator's fleet the selection to DITCH is achieved by pushing in and rotating the switch in an anti-clockwise direction. All other aircraft in the fleet are fitted with a selector where the selection of DITCH is achieved by pulling out the selector switch and then rotating in an anti-clockwise direction. The operator removed the pressure control panel and returned it to the vendor for a check of the switch depression and spring tension settings. Pending the results of these tests the operator is examining the replacement of the push actuated switches with the pull actuated switches. An Operational Notice has been issued to all pilots reminding them of the position and operation of the selector, alerting them to take particular care when getting in and out of the right pilot's seat in flight.