Ahead of today's ruling on Prime Minister Hun Sen's defamation suit against opposition lawmaker Mu Sochua, a prominent rights groups said it was doubtful Phnom Penh Municipal Court could pass fair judgement on a case that legal observers say has come to define government interference in the judiciary.

In a statement Monday, the Hong Kong-based Asian Human Rights Commission said that due to the current state of the courts, a verdict in favour of Hun Sen was largely a foregone conclusion.

"Executive control of the court is an established fact, and it is known that the court lacks independence," the group said.

It added that "almost all prosecutors and judges" were affiliated with the ruling Cambodian People's Party, saying that any verdict against Sam Rainsy Party lawmaker Mu Sochua would "not carry any authority" in a proper court of law.

Mu Sochua told the Post that she saw her case as a test for the independence of Cambodia's court system.

"Tomorrow, the court has an opportunity to show its independence. If the court decides against me, the court is not independent," she said Monday.

The SRP and Human Rights Party say that they have prepared a joint statement but will wait for today's verdict before criticising the courts.

"We will issue a joint statement, [but] its contents will be different depending on the verdict," HRP spokesman Yem Ponharith said.

But Prime Minister Hun Sen's lawyer Ky Tech, who is prosecuting the case, said that if the SRP and Mu Sochua complain about a guilty verdict, they are themselves preventing the court from being independent.

"Normally, everyone says the court is not independent when he loses a case," he said. "But if he wins, he says the court is independent."

Hun Sen sued Mu Sochua for defamation after she filed her own defamation suit against him, claiming he insulted her during a speech in Kampot province in early April.