A Cambodian man has been jailed for three years for insulting the king in Facebook posts, the second known conviction under a new law enacted last year.

Phnom Penh Municipal Court spokesman Y Rin says Ieng Cholsa was sentenced to three years in prison and ordered to pay five million riels ($A1740) on Wednesday.

The Facebook posts, which the court found had insulted King Norodom Sihamoni, were uploaded in June last year, Y Rin said.

The 26-year-old chicken vendor called the monarch “useless” and a “dog king” in the posts, according to local media.

Cambodia’s ‘lese majeste’ law was unanimously adopted by parliament in February last year.

Rights groups expressed concerns at the time that the law, which is similar to legislation in neighbouring Thailand, could be used to silence government critics.

Last October, a court in the Northern Province of Siem Reap jailed a member of the dissolved opposition Cambodia National Rescue Party under the law.

The supreme court dissolved the Cambodia National Rescue Party in November 2017 at the government’s request after it was found guilty of plotting a US-backed coup.

Washington and the CNRP have denied any such plot to overthrow the government.

The Cambodian prime minister, Hun Sen, and his ruling Cambodian People’s party won a general election that critics said was only possible because the opposition had been dissolved.

A veteran member of the CNRP broke ranks with his colleagues on Wednesday and became the first to apply for a government-offered lifting of a ban on engaging in political activity.

Kong Korm was one of 118 members of the CNRP who were banned from politics for five years when the group was dissolved.

Cambodia’s courts are widely considered to be under the influence of long-serving Prime Minister Hun Sen’s government.

Many of the banned opposition politicians fled Cambodia in fear of arrest, and restoration of their political rights alone would appear to leave them in the political wilderness. There are no guarantees that new legal actions would not be taken against them in the courts.

The BBC recently reported that the UN urged Thailand in 2017 to amend its harsh lese majeste law.

The Office of the UN High Commissioner for Human Rights protested the high rate of prosecutions and the disproportionate sentences for offences that in other countries are permitted by freedom of speech laws.

In one extreme Thai case, a 48-year-old tour guide operator, Pongsak Sriboonpheng, was sentenced to 60 years in prison, 10 years for each of the Facebook posts in which he criticised the monarchy. He pleaded guilty and the sentence was halved to 30 years.

Lese majeste laws are currently in place in Denmark, Germany, Iceland, the Netherlands, Spain, Kuwait, Jordan, Saudi Arabia, Morocco and Malaysia