Image copyright AFP Image caption Cambodia's King Norodom Sihamoni pictured with Queen Mother Norodom Monineath

Cambodian police plan to arrest three suspects who are accused of doctoring an image of the country's sovereign to place him in a gay porn scene, along with the words "Cambodia King is gay".

The picture was posted on Facebook on Christmas Day, and seen in both Cambodia and Thailand.

It is being viewed as a rare public insult against King Norodom Sihamoni.

The monarch, who became king in 2004, is largely respected by the public and seen as being above politics.

While the monarchy in neighbouring Thailand is protected by a lese majeste law that outlaws any criticism, there is no similar law in Cambodia.

However, the country's constitution says the king is "inviolable" - or too significant to be treated with disrespect.

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A spokesman for Cambodia's interior ministry confirmed that an investigation had been launched.

General Khieu Sopheak told the AFP news agency: "We have got orders to arrest them. If we don't take action against them, more people might follow their act."

He refused to specify which law the three suspects might have broken, saying: "The king represents the whole nation, and they are insulting the king, which is like they are insulting the whole nation."

Image copyright AFP Image caption In December, the king welcomed visiting Philippines President Rodrigo Duterte to the Royal Palace in Phnom Penh

Two of the suspects are believed to be in Cambodia, and the third in Thailand.

Gen Sopheak said the authorities could seek help from Bangkok.

Cambodia's kings once enjoyed a semi-divine status, but the role is mostly ceremonial nowadays.

Observers say 63-year-old Sihamoni has ruled quietly as a constitutional monarch, fulfilling his duties while keeping out of the country's turbulent domestic politics.

This is an arrangement that has suited the country's wily prime minister, Hun Sen, one of the world's longest serving heads of government.

The king is an accomplished linguist who speaks English, French and Czech. Before becoming sovereign he spent much of his adult life abroad, pursuing a career in classical dance.

He is unmarried, and has no children.

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When King Norodom Sihamoni inherited the throne from his father, Norodom Sihanouk, the BBC reported:

"Few people had heard of Norodom Sihamoni before Cambodia's former king, Sihanouk, announced his abdication in early October.

"But within days of the announcement, the 51-year-old ex-ballet dancer was thrust into the limelight as the most likely candidate to become the impoverished nation's next king.

"The modest, music-loving bachelor was initially reluctant to take the throne, but said he was willing to do so if asked by Cambodia's official Throne Council.

"Although he has represented his country as ambassador to the United Nations cultural organisation, Unesco, he has so far played no part in Cambodia's often chaotic politics.

But his distance from political wrangling led him to be seen by both Sihanouk and Prime Minister Hun Sen as the most suitable candidate to succeed to the throne, a position which in recent years has become largely symbolic."