Cambodia's former opposition leader faces royal insult charges punishable by a jail term, according to a court summons, the first prominent figure to be accused under the new law.
The adoption of the law in February triggered alarm from rights groups who warned it would become another tool for Prime Minister Hun Sen to stifle dissent.
The strongman premier is seeking to prolong his 33-year grip on power in a general election next month and has used the courts to cripple his opponents.
Since the lese majeste law was approved three people have been charged for Facebook comments deemed insulting to King Norodom Sihamoni, a symbolic head of state who has steered clear of politics since taking the throne in 2004.
Now Sam Rainsy, the former leader of the banned Cambodia National Rescue Party (CNRP), is under investigation after calling into question the authenticity of a public letter from Sihamoni urging people to vote.
A prosecutor on Wednesday issued a warrant summoning Rainsy to appear in court next month to answer questions over a "case related to insulting the king of Cambodia on a Facebook page", according to a copy of the document seen by AFP.
On June 6 Rainsy wrote a post on his widely followed page calling Sihamoni's letter a "forgery" and "worthless" and speculating that it could even have been produced under duress.
The palace denied the accusation.
Rainsy faces up to five years in prison if found guilty.
But he is not expected to appear in court as he lives in self-exile in France to avoid the mounting charges and convictions against him, which he says are politically motivated.
His opposition party was dissolved under a court order last year as part of a sweeping crackdown.
In recent weeks former opposition lawmakers -- mostly those who fled the country in the wake of the crackdown -- have urged voters to boycott July's poll in protest.
Western democracies have withdrawn support for the July vote and the US sanctioned the head of Hun Sen's bodyguard unit last week for his alleged role in human rights abuses.