Mr Rainsy says the charges are an attempt to silence the opposition
The Cambodian government has filed a new lawsuit against leading opposition figure Sam Rainsy.
The government accuses Mr Rainsy of forging public documents and spreading false information about a border dispute with Vietnam.
Mr Rainsy, who is living in exile, was given a two-year jail term last month for a political protest in which markers along the border were uprooted.
He could face up to 18 years in prison if found guilty of the latest charges.
"The lawsuit involves forging public documents and publicising disinformation related to the forgery of a map in order to manipulate the public over the border issue with Vietnam," government lawyer Ky Tech told the AFP news agency.
In January, Mr Rainsy was given the two-year jail term in his absence for encouraging villagers to uproot the border markings.
He did not attend the hearing, saying in an e-mail believed to have been sent from France, that the case against him was politically motivated.
"The court in Cambodia is just a political tool for the ruling party to crack down on the opposition," he said.
"I will let this politically subservient court prosecute me in absentia because its verdict is known in advance."
Cambodia and Vietnam officially began demarcating their contentious border in September 2006, in a bid to end decades of territorial disputes.
The 1,270-kilometre (790-mile) border has remained essentially unmarked and vague since French colonial times, with stone markers and boundary flags having disappeared, while trees once lining it were cut down.
Mr Rainsy's party accuses the Cambodian government of ceding territory to its larger and more powerful neighbour.