PHNOM PENH (REUTERS) - Cambodian opposition party leader Kem Sokha is set to go on trial on treason charges on Wednesday (Jan 15) in a closely watched case as the European Union mulls suspending trade benefits over the South-east Asian nation's human rights record.

It could take up to three months to deliver a verdict, defence lawyers said on Tuesday, meaning the case could still be ongoing by the time the European Union (EU) makes a final decision on its trade policy next month.

Kem Sokha was arrested in 2017 and his opposition Cambodia National Rescue Party was banned ahead of a 2018 election in which the party of longtime leader Hun Sen won all the parliamentary seats.

Kem Sokha was freed from house arrest in November, but a ban on his engaging in political activity was kept in place.

The treason charges stem from accusations that Kem Sokha was conspiring with the United States to overthrow Mr Hun Sen. He denies the charges, saying they are politically motivated.

Many other opposition figures have fled into exile and have accused Mr Hun Sen, who has ruled Cambodia for 35 years, of establishing a one-party state.

One of Kem Sokha's attorneys, Mr Chan Chen, said a court official had informed him that the trial may take up to three months.

The EU, which accounts for nearly half of Cambodia's exports, will decide in February whether or not to remove Cambodia from its Everything But Arms trade scheme over its crackdown on political expression.

The only evidence presented against Kem Sokha in public so far is a comment he made at a public meeting in which he said he was getting advice from unspecified Americans on election strategy ahead of a 2013 ballot.

Kem Sokha's daughter, Ms Monovithya Kem, called the trial against her father a "farce" as rights groups urged the case to be dismissed.

"We hope that he will be acquitted if the court has any interest at all in starting to demonstrate its independence," his daughter said.

Amnesty International called the charges "bogus", saying authorities have not presented a shred of credible evidence to support them.

Ministry of Justice spokesperson Chin Malin declined to comment and Phnom Penh Municipal Court spokesman Y Rin couldn't be reached for comment on Tuesday.