Cambodia's parliament last week adopted a law that forbids insulting the monarchy, along with other amendments to the constitution on the rights of voters, prompting concerns from rights groups who say they could be used against government critics.
"We urge the government of Cambodia to carry out a rigorous and thorough reassessment of the draft amendments to ensure they comply with international human rights laws and standards," Rhona Smith and David Kaye, both UN Special Rapporteurs, said in a statement.
The UN experts added that the proposed amendments use "broad terminology and would need more precise language to meet international standards".
The UN criticism is unlikely to carry any weight as long-serving Prime Minister Hun Sen has been a long-term critic of the organisation, especially over its criticism of his government and its rights record.
The United Nations helped bring peace to the country after more than a decade of war in the 1980s, which followed the brutal 1975-79 "Killing Fields" rule of the Khmer Rouge.
Government spokesman Phay Siphan could not be reached for comment on Tuesday.
The lese-majeste law stipulates that those found guilty would face between one and five years in prison and a fine of between $500 and $2,500.
King Norodom Sihamoni is officially Cambodia's head of state, but Hun Sen has ruled the Southeast Asian country for more than 33 years.
Hun Sen has come under criticism from Western countries and rights groups for a crackdown on government opposition ahead of elections in July, at which he is expected to extend his 33-year rule.
A Cambodian court dissolved the opposition Cambodia National Rescue Party last year, following the arrest of its leader Kem Sokha on treason charges, which he says were politically motivated.