Immigration authorities in Malaysia have detained Mu Sochua, vice president of the banned opposition Cambodia National Rescue Party (CNRP), four sources with direct knowledge of the matter told Reuters news agency on Thursday.
The sources, one of whom is with the Malaysian government, declined to be named.
Malaysia’s immigration department did not immediately respond to Reuters new agency’s questions.
In late October, Mu Sochua was also denied entry to Thailand and was sent back to Malaysia.
The news comes just a day after Malaysian authorities acknowledged on Wednesday that they have detained two Cambodian opposition activists late on Tuesday.
Cambodia has arrested at least 48 opposition activists this year for allegedly plotting to overthrow the government before the planned return from self-exile of Sam Rainsy, founder of the dissolved CNRP, on Saturday.
Hun Sen’s government deployed troops along its borders in response to Rainsy’s announcement of his planned return.
Rainsy fled to France four years ago following a conviction for criminal defamation in which he was ordered to pay $1m in compensation. He also faces a five-year prison sentence in a separate case.
He has previously said it was legitimate to seek to topple Hun Sen because the prime minister has created a one-party state and was not prepared to hold free and fair elections.
On Wednesday, Thai Prime Minister Prayuth Chan-ocha said that Rainsy would not be allowed to enter Thailand on his way to Cambodia.