Remarks: Flight CX720 was scheduled bound for Hong Kong. Just before aircraft closing it's doors, an undocumented passenger illegaly boarded this aircraft was spotted standing in the aisle. After all necessary steps he was escorted off the aircraft. All passgers have to get off the aircraft as well and re-screening through security for re-boarding. This illegal passenger's luggage was off-loaded from the aircraft. This flight was delayed 2 hours before it departed.