The flight was part of a type endorsement. After completing the upper air work sequences, five circuit and landings were carried out without incident, although the fifth landing was reported as being heavier than normal. On the downwind leg of the next circuit, both pilots reported that the gear was found to be still extended when the pre-landing checks were commenced. The gear lights continued to indicate a down and locked situation, but shortly after touchdown the left maingear began to retract. The aircraft veered to the left and the nosegear retracted prior to the aircraft coming to rest, ten metres off the side of the runway. The pilots reported that, after they had vacated the aircraft, they observed the gear selector to be in the up position. Both pilots reported that they had not touched the lever during the landing. The only fault found with the undercarriage system was that the gear selector switch was worn such that the selector could be moved from "DOWN" to "UP" without overcoming the mechanical lock. The pilots had relied on the green light indication for gear position during the pre-landing checks. Neither had physically checked the position of the selector lever. The reason the wearing in the switch was not detected during maintenance and servicing of the aircraft was not established.