Cambodia's election commission on Monday disqualified the sole opposition Candlelight Party from contesting elections in July over its failure to submit proper registration documents.
Other parties have signed up to contest the general election, but Candlelight's disqualification means the ruling Cambodian People's Party (CPP) looks set to run virtually unopposed.
Some activists and diplomats have warned against what they call long-serving Prime Minister Hun Sen's actions to suppress opponents, fearing they could undermine the democratic process in the Southeast Asian country.
Asked for comment on Candlelight's disqualification, CPP spokesperson Sok Eysan said the election would be free and fair, adding that more than 10 parties had registered.
Hun Sen has previously said the CPP will dominate politics for up to 100 years.
Just over a year old, Candlelight is a reincarnation of the Cambodia National Rescue Party (CNRP), a popular opposition that the Supreme Court disbanded in 2017 ahead of an election that was swept by CPP.
Scores of former CNRP members have been detained or convicted of crimes, many in absentia having fled into exile amid Hun Sen's sweeping crackdown on critics.
Candlelight deputy president Son Chhay said it would appeal to the constitutional court.
"We have one week to do so," he said in a text message.
Human Rights watch last month accused Cambodia's government of stepping up attacks on the opposition with rhetoric that had led to assaults on Candlelight members.
It took aim at Hun Sen for what it said were warnings against criticising his government ahead of the election.
In an April 24 statement, it said foreign governments should send a clear message that "dismantling opposition parties and disqualifying, assaulting, and arresting their members before election day means that there won't be any real election at all".
The government has denied targeting its opponents, saying legal cases against them were enforcement of the law.