By Prak Chan Thul
PHNOM PENH (Reuters) - Cambodia's parliament voted on Monday to allow a court investigation into an opposition leader accused of procuring a prostitute, a vote that could push the country closer to political turmoil.
The case of the opposition leader, Kem Sokha, and his alleged assignation has dominated politics for weeks and has raised tension in the run-up to a 2018 election that could prove to be veteran Prime Minister Hun Sen's most serious test at the ballot box.
Opponents of Hun Sen say the self-styled strongman is using the judiciary to neutralize his opponents. The government denies that.
Kem Sokha is acting chief of the main opposition Cambodia National Rescue Party (CNRP).
Party leader Sam Rainsy, a former finance minister, lives in self-exile to avoid arrest for an old defamation case he was pardoned for. His party has denounced the new warrant as politically motivated.
All 68 members of parliament from Hun Sen's ruling Cambodian People's Party voted to allow a court to proceed with an investigation into Kem Sokha stemming from a secretly recorded, and leaked, telephone conversation between him and a woman who the government says he was having an illicit affair with.
Prostitution was made illegal in Cambodia in 2008 even though it is widespread.
Kem Sokha has neither confirmed nor denied having an affair with the woman and has dismissed the action against him as politically motivated. He has ignored court summonses, most recently last Thursday.
Trade unions allied with the CNRP have called for protests if Kem Sokha is arrested.
"The CNRP wants Cambodia to be ruled by rule of law and a multi-party democracy," Son Chhay, a senior CNRP lawmaker, told a news conference.
"We will continue our fight."
The party said Monday's vote was unconstitutional.
The last election in 2013 marked Hun Sen's toughest electoral challenge in three decades of rule and the opposition is expected to mount a sterner test next time as younger voters seek change.
Hun Sen has warned that an election victory for the opposition would see a return to civil war.
The United Nations on Sunday voiced alarm at the escalating tension between Hun Sen's party and the opposition, in particular the arrest or attempted arrest of politicians.
(Editing by Amy Sawitta Lefevre and Robert Birsel)