The favourite dog of Thailand's ailing monarch has died, days after a man was arrested under the kingdom's strict royal defamation laws for allegedly making a satirical online remark about the beloved canine.

The dog, called Tongdaeng (Copper), became both a household name and a publishing sensation in Thailand after King Bhumibol Adulyadej adopted her as a stray puppy and penned a heartfelt book about her attributes in 2002.

She was praised for her loyalty and obedience, with the book widely interpreted at the time as a parable about how Thais should live and remember their place within the kingdom's rigid society.

Her death was announced in a statement from the veterinary faculty of Kasetsart University late Monday.

"While Khun Tongdaeng was sleeping and relaxing, she died peacefully on 26 December 2015 at 11:10pm at Klai Kangwon Palace," the statement read.

It said she had been ill for the last few years and was just over 17 years old when she passed away.

"Khun" is an honorific in Thailand, roughly translating to "ma'am", and was frequently used by officials and local media when referring to Tongdaeng.

Her death was splashed across Thai newspapers Tuesday, with local media reporting that the king had been informed.

The dog regularly featured in palace photographs while the book about her outsold bestsellers such as Harry Potter in Thailand.

A privately funded animated film based around Tongdaeng and her attributes is currently showing at Thai cinemas.

In his book, Bhumibol described Tongdaeng as a "respectful dog, with proper manners" who was "humble and knows protocol".

"She would always sit lower than the king," the book added.


Criticism banned

King Bhumibol, 88, has spent much of the last two years in hospital and is rarely seen in public. But the world's longest-serving monarch remains widely revered in Thailand where his economic and social teachings are extensively promoted.

The monarchy is also shielded from any debate and criticism by one of the world's harshest royal defamation laws.

Anyone convicted of insulting the king, queen, heir or regent, can face up to 15 years in jail on each count.

Analysts say uncertainty as the king's reign enters its twilight years is a major factor in the political chaos that has beset Thailand for the much of the last decade, as competing elites jostle for influence.

Lese majeste prosecutions have soared since the army, which styles itself as the champion of the monarchy, grabbed power in a coup last year.

The boundaries for what counts as a royal insult have also expanded dramatically.

Earlier this month Thanakorn Siripaiboon, 27, was arrested for allegedly making a "satirical" Facebook post about the king and his dog, according to his lawyers.

As is often the case in lese majeste cases, the authorities did not give details on what the post said.

Thanakorn also faces lese majeste, sedition and computer crimes charges for clicking "like" on a doctored photo of the king and sharing it, plus an infographic on a growing corruption scandal engulfing the military.