Washington hit out after Cambodia's Supreme Court -- effectively controlled by strongman premier Hun Sen -- outlawed the Cambodia National Rescue Party and banned more than 100 of its politicians in a ruling blasted by a rights groups as the "death" of the nation's democracy.
The European Union echoed Washington's concerns over the move which means Hun Sen's CPP party can now run in next year's polls essentially uncontested.
The verdict is the culmination of a methodical strangling of dissent in Cambodia that began after the CNRP nearly unseated Hun Sen -- who has ruled for 32 years -- in the last national election in 2013.
A government clampdown has ratcheted up in recent months, with CNRP president Kem Sokha thrown into jail and charged with treason in September.
The United States blasted Thursday's ruling as a setback for democracy in Cambodia, calling for the government to "undo its recent actions against the CNRP, release imprisoned CNRP leader Kem Sokha, and allow opposition parties, civil society and the media to maintain their legitimate activities," the White House said in a statement on Thursday in Washington.
It added that it would pull support for Cambodia's National Election Committee ahead of next years' vote.
"On current course next year's election will not be legitimate, free, or fair," the statement said.
'Not legitimate'
Washington joined the European Union in condemning the decision, which follows a months-long crackdown on dissent the country with media outlets shuttered, journalists jailed and activists harassed.
The EU warned Thursday that next year's elections are stripped of credibility with the CNRP now pulled from the race.
"An electoral process from which the main opposition party has been arbitrarily excluded is not legitimate," said a statement from a EU spokesperson.
"A situation in which all parties, including the CNRP, their leaders and their supporters are able to carry out freely their legitimate functions, must be swiftly restored," it added, saying "respect of fundamental human rights" is a prerequisite Cambodia's duty free access to the bloc's markets.
The CNRP said Thursday it "still considers itself to be a legitimate party with a mandate from half of the Cambodian population," though more than half its 55 lawmakers have fled the country in the midst of the crackdown.
The ruling was widely expected from a court closely aligned with Hun Sen's CPP party.
The CNRP was accused of a US-backed plot to overthrow the government in allegations which were ridiculed by the US and NGOs.
The former Khmer Rouge defector, who has staked his reputation on bringing stability and growth to Cambodia after decades of civil war, said Thursday next year's elections would be held as scheduled.
The premier has a long history of undercutting his rivals through well-timed crackdowns and dubious court cases.
But observers say the current climate of repression is harsher and longer-lasting than previous clampdowns, with Hun Sen foregoing even the pretense of respecting human rights and a free press.
In addition to assaults against the CNRP, his government has in recent months shut down a series of outspoken NGOs and independent news outlets -- including the respected Cambodia Daily.
This week, two former reporters from the US-based Radio Free Asia were arrested and accused of supplying a foreign state with information that threatens national security.
Analysts say Hun Sen has been emboldened by financial backing from Beijing, which has lavished the poor country with investment that has made it less dependent on aid from Western democracies.
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