PHNOM PENH--Cambodia's prime minister lifted a ban Wednesday on the return to the country of his main rival, daring him to risk imprisonment under an outstanding criminal conviction.

The order issued by Hun Sen allowing the return of Sam Rainsy, for two decades his most persistent political opponent, comes as attention is being turned to next year's general election. Hun Sen is one of Asia's most cunning politicians, wielding either carrot or stick to coopt or vanquish his foes.

In nationwide local elections earlier this month, Hun Sen's ruling Cambodian People's Party won most constituencies but received a weak majority of the popular vote, where the opposition Cambodia National Rescue Party made gains.

Hun Sen has repeatedly sought to sideline Sam Rainsy, especially by pursuing defamation cases against him. Sam Rainsy has stayed abroad since late 2015, when a conviction he believed was covered by an earlier pardon was revived. Last October, he was officially banned from re-entering the country, and several other cases against him were launched or concluded.

The new order forces Sam Rainsy to make good on a vow to return if the ban was lifted or risk losing face, especially among his own supporters.

Sam Rainsy's reluctance to return and risk prison as Hun Sen put great pressure on his party caused dismay among some of his colleagues, weakening the charismatic politician's leadership profile. His absence also encouraged a latent rivalry with his own party deputy, Kem Sokha, who became acting leader in his absence.

Kem Sokha became the opposition party's official leader in March after a law passed by the government-controlled legislature put the party in peril of being dissolved if its leaders had criminal convictions. Sam Rainsy stepped down as leader and formally resigned his party membership.

Hun Sen's order lifting the re-entry ban came a day after Sam Rainsy declared his intention to challenge the prime minister in the 2018 general election.

For next year's polls to be fair and allow a real challenge to the prime minister, "I would like request Mr. Hun Sen to be brave and do not ban me from returning in order to challenge him," he said in an interview with U.S.-government funded Radio Free Asia.

Otherwise, Hun Sen would be like a boxer alone in a ring without an opponent, Sam Rainsy said.

Asked specifically if he would return even if he faced prison time, he told his interviewer that if he were allowed back in, he would go.

A spokesman for the Cambodia National Rescue Party, Yim Sovann, welcomed the order dropping the ban.

"This is a good sign, but we are not sure what reason is behind the issuance of this letter. We will keep an eye on the issue," he said, adding that he believed Sam Rainsy would return to Cambodia despite the risk of prison.

He added, however, that the party had not yet decided on its candidate for prime minister next year.

"Once he comes, we will discuss more about the candidates, but the main thing for now is how to get him to Cambodia before the election in 2018," Yim Sovann said.