Opposition leader Sam Rainsy yesterday accused the ruling Cambodian People’s Party of launching a “constitutional coup” by stripping him of his parliamentary immunity after a court issued a warrant for his arrest last week.

Despite earlier promises, Rainsy has remained abroad since the issuance of the warrant – which seeks to implement a two-year prison sentence for defaming Foreign Minister Hor Namhong in 2008 – saying he was allowing time to negotiate a peaceful solution to the matter.

But yesterday’s strongly worded comments came just as the Cambodia National Rescue Party leader was offered a thorny choice by an unofficial intermediary that could potentially allow for his return to Cambodia.

Soy Sopheap, an influential media baron who has helped negotiate political settlements in the past, said yesterday that he told the CNRP that, should Rainsy return to face the court, he could be offered a reduced sentence.

Alternatively, Sopheap added, Rainsy could pen a public letter of apology to Namhong from self-imposed exile that “recognises the Cambodian People’s Party of [Prime Minister] Hun Sen as the national liberator from the Khmer Rouge”.

“This is a very good choice for him to avoid the enforcement of the arrest warrant,” Sopheap, who said he was acting without the authorisation of the ruling CPP, said of the latter option.

Suos Yara, a CPP spokesman, said that there was no official plan to strike a deal with Rainsy and that the party was not aware of attempts at negotiation by Sopheap.

The Phnom Penh Municipal Court issued the warrant on Friday, less than 24 hours after Hun Sen threatened legal action against him following comments Rainsy made to supporters in Japan where he questioned the CPP’s commitment to democracy.

He postponed a planned flight to the Kingdom on Monday, citing safety concerns. He said would return to Cambodia “in the next few days”.

The charges stem from comments Rainsy allegedly made in 2008, suggesting that Foreign Minister Hor Namhong had been responsible for crimes at the Boeung Trabek prison during the Khmer Rouge regime of 1975 to 1979.

A court in France, where Namhong filed a second defamation suit in 2008, found Rainsy guilty the following year. The verdict was upheld in 2010, but in April 2011, the French Supreme Court overturned the verdict.

Rainsy on trial: view an interactive timeline of the opposition leader's ongoing legal battles

In a statement issued yesterday morning on Facebook, Rainsy said that his expulsion from the National Assembly just before the issuance of the arrest warrant amounted to “a constitutional coup”.

He added that the “brutal moves” against the CNRP were intended to take Cambodia “back to a one-party system”, and were an attempted “derailment of the democratisation process”.

But opposition spokesman Eng Chhay Eang said the CNRP was open to negotiations which could allow for Rainsy’s “peaceful return” to Cambodia.

The purported negotiations come during one of the most fractious political periods since the 2013 national elections.

The warrant and expulsion from parliament of the party president followed the beating of two opposition lawmakers outside the National Assembly in late October. Three military officials have since been charged over the assault.

The attackers were part of a pro-CPP demonstration outside the National Assembly calling on Kem Sokha, the CNRP deputy leader, to resign from the vice presidency of the legislature. He was stripped of the position by CPP lawmakers days later.

UN Secretary General Ban Ki-moon yesterday expressed concern for the deteriorating political situation, calling the crackdowns on the opposition “worrisome developments”.

And in a statement, 19 non-governmental groups led by democracy watchdog Comfrel condemned the years-old defamation verdict against Rainsy.

“The court’s verdict was clearly political and an example of the government controlling the judicial system as a tool to disrupt the opposition’s voice in Cambodia,” the statement said.

CPP spokesman Sok Eysan dismissed the NGOs’ comments, saying the groups were only interested in “looking into every dark corner” and ignoring government successes.