Royal palace officials said King Sihanouk's former secretary Prince Sisowath Thomico left Cambodia on Tuesday in a hurry.
Prime Minister Hun Sen on Monday said that he was considering filing a defamation lawsuit against the prince for accusing him of giving Koh Tral to Vietnam. PM Hun Sen said even though the prince is former King Sihanouk's cousin, he does not have immunity.
In a letter posted on former King Sihanouk's website, Prince Thomico reportedly said Koh Tral was occupied by Vietnam since 1982. Prime minister Hun Sen said his government never gave Cambodian land to any foreign countries.
A number of border activists have either left the country or are in hiding for fear of being arrested or imprisoned.
The US Embassy and civil society officials have condemned Hun Sen's measures to silence his critics. Government spokesman Khieu Kanharith said the government only wants the dissidents to speak the truth.