An air carrier pilot rejected the takeoff after another aircraft entered the runway at CLT. The aircraft were within four feet of one another when the reject was completed.

Narrative

We were cleared for takeoff in CLT and began the takeoff roll. After the Captain called for thrust to be set. I set the thrust and we both monitored the takeoff. At approximately 85 KTS we noticed an aircraft starting to taxi on to runway. I commented that the aircraft appeared to be entering the active runway and when it became clear that the aircraft was entering the runway the Captain confirmed what I was also seeing. The Captain rejected the takeoff and I made the standard call outs as we slowed. Our aircraft came to a complete stop approximately 3 or 4 feet away from the intruding aircraft. The intruding aircraft then exited the runway on the next available left taxiway and we set the parking brake. The Captain advised the passengers to remain seated and I pulled out and read the rejected takeoff QRH. I advised the tower we would need to exit the runway and contact Maintenance. We then exited the runway without any further incident. The intruding aircraft was initially holding short of the active runway. After we were cleared for takeoff I do not know why the aircraft entered onto the runway. It was holding short at an intersection; and although I am speculating it may be the Controller may have thought it was holding short full length.