Despite resolving a possible 12 passenger discrepancy with load planners prior to departure; the flight crew of an A320 received an ACARS message as they neared cruise advising there were actually 48 more passengers on board than confirmed in the earlier communication.

Narrative

Our paperwork had a preliminary passenger count of 99. The passenger service manual count was 87. The Captain noticed the 12 passenger discrepancy and requested an explanation. The explanation given was that passengers had boarded an earlier flight thus reducing our number of passengers. The Captain then asked twice if a manual count was recorded and was assured both times we had 87 passengers on board.We pushed back two minutes early and were airborne shortly thereafter. Nearing cruise we received an ACARS from Dispatch advising us that the number of passengers onboard was 136. The Captain then requested a passenger count from the Lead Flight Attendant. That count totaled 133 plus 1 child. The Captain changed our weights and performance data to reflect the appropriate numbers for landing.Had either the preliminary count or the actual count been correct we would have had a greater chance of catching the error. As it turned out; the two incorrect numbers were close enough to accept the false reason given to us that 87 passengers were onboard.