The opposition Sam Rainsy Party, Human Rights Party, and Norodom Ranariddh Party told the king that one million registered voters were cut from the rolls by National Election Committee (NEC) officials.
“This act is an intentional mistake which is committed by election officials,” the parties said in the joint letter, relased late Wednesday.
“This is a serious mistake committed by NEC officials at all levels which prevented at least one million people from voting,” the letter said, asking the king to find a way for them to cast ballots before the announcement of official election results.
Prime Minister Hun Sen’s ruling Cambodian People’s Party has claimed a landslide victory in Sunday’s election, saying it captured at least 90 of the 123 seats in parliament. Final results of the poll are expected in September.
More than six million out of 8.1 million eligible voters cast ballots in the poll.
European Union election observers have said only about 50,000 could not find their names registered when they attempted to vote.
The NEC has denied opposition allegations that voters were left off the rolls on purpose.
International monitors said earlier this week that the election was flawed and did not meet key standards, despite a more peaceful campaign and improvements in the electoral process.
The opposition parties have rejected the results of the election and demanded a re-run of the poll.