The Constitutional Council on Tuesday rejected the request for a ballot recount in Svay Rieng province, claiming the Sam Rainsy Party showed false evidence in its case.
The Sam Rainsy Party had requested a recount in 650 ballot stations, citing a difference in counting results between the party and the National Election Committee results.
Opposition leader Sam Rainsy walked out of the hearing, telling reporters the proceeding was like a movie "scene" and the decision by the Council had been made in advance.
A recount would have showed cheating by the NEC, he said. NEC representatives denied the accusations during the hearing.
The Council should have taken more time to investigate before declaring its decision, as it is the last place for complaints to be heard, said Hang Puthea, executive director of the Neutral and Impartial Committee for Free and Fair Elections in Cambodia.