PHNOM PENH (Reuters) - Cambodia’s main opposition party will boycott a parliamentary vote to strip their detained leader of immunity on Monday and will instead go to Kem Sokha’s jail to demand his release, one of his deputies said on Sunday.

FILE PHOTO: Cambodia's opposition leader and President of the Cambodia National Rescue Party (CNRP) Kem Sokha talks during an interview with Reuters in Prey Veng province, Cambodia May 28, 2017. REUTERS/Samrang Pring/File Photo

Kem Sokha, the head of the Cambodian National Rescue Party (CNRP), was arrested a week ago and charged with treason for allegedly plotting to win power with the support of the United States, escalating a crackdown on critics of Prime Minister Hun Sen and independent media ahead of a general election next year.

Two aid groups run by the publisher of the Cambodia Daily, shut a week ago in a dispute over a crippling tax bill, said they were suspending work after their accounts were frozen.

Parliament is due to vote on whether to remove the immunity from prosecution which Kem Sokha gets as an elected member of parliament. The ruling Cambodian People’s Party’s (CPP) majority means the motion is certain to pass anyway.

CNRP deputy leader Mu Sochua said the parliament vote was illegal.

“We can’t accept this. We will demand that Kem Sokha, who has not done anything wrong, be released,” she said, adding members of parliament would hold a protest at the prison where he is being held near the border with Vietnam.

CPP spokesman Sok Eysan said the opposition boycott would be unconstitutional, but the CPP had enough votes to strip Kem Sokha of his immunity anyway.

Hun Sen, a 65-year-old former Khmer Rouge commander, has ruled Cambodia for more than 30 years and said last week he planned to stay in power for another decade.

Next year’s election had been expected to be his toughest electoral test, but Western countries and human rights groups have raised doubts as to whether the vote will be fair.

They have criticized Kem Sokha’s arrest, but China has given its backing to close ally Hun Sen.

The evidence presented against Kem Sokha so far is a video recorded in 2013 in which he discusses a strategy to win power with the help of unspecified Americans. His lawyers have dismissed the evidence as nonsense and said he was only discussing election strategy.

In recent weeks, Hun Sen has also expelled the National Democratic Institute, a non-governmental organization that promotes democracy, and ordered 19 radio stations off the air.

The independent English-language Cambodia Daily shut last week after being given a month to pay a $6.3 million tax demand.

Japan Relief for Cambodia and World Relief for Cambodia, aid groups financed by the paper’s publisher, said their accounts had been frozen and they would have to suspend building schools and teaching English and computing to thousands of pupils.