"Involvement of a high-ranking member of the royal family in politics, in whatever way, is considered an act that defies the nation's traditions, customs and culture, and therefore is considered extremely inappropriate".
Prayuth also accepted on Friday the nomination as a candidate for prime minister from his military-backed party.
As I have observed before in these pages, common references to Thailand as a democracy and a focus on contemporary developments including the May 2014 coup that brought the junta-led government under Prayut Chan-o-cha to power and the March 2019 expected election date can obscure more complex realities in the country's politics.
Thailand's King Maha Vajiralongkorn said on Friday (Feb 8) his elder sister's announcement she is running for prime minister in the March 24 election is "inappropriate" and unconstitutional, likely sinking her candidacy for a populist opposition party.
Because Thailand is a constitutional monarchy, the king and his immediate circle are not supposed to involve themselves directly in politics. Though Ubolratana does not have any royal titles and is not covered under lese majeste laws as of now - she relinquished her royal status back in 1972 - it is hard to imagine that her status would not affect how polls would be conducted.
In a strongly worded statement, the king said senior royal family members should remain above politics and be politically neutral.
Thailand has some of the world's toughest royal defamation "lese-majeste" laws but technically the princess is not covered by them. The army eventually ousted him from the prime minister's office in a 2006 coup.
It remains possible Prayut's backers could mount a legal challenge to Ubolratana's candidacy by claiming she is violating laws created to keep the monarchy out of politics. The Thai princess will stand for a party allied to controversial ex-prime minister Thaksin Shinawatra, according to registration papers.
Thaksin's well-funded political machine returned to power twice since and is seen as the key element in arranging for Ubolratana's selection because she cast her lot with a Thaksin-affiliated party.
For the first time in the 86-year history of Thailand's constitutional monarchy, a member of the nation's royal family has run for prime minister.
Ubolratana is the lone candidate put forth by the Thai Raksa Chart party, whose members include Thaksin loyalists.
In 2016, Thais voted to approve a new constitution created by the country's military leaders, which was designed to perpetuate military influence and block Mr Thaksin's allies from winning another election.
"We are in new territory", said Kevin Hewison, an emeritus professor at the University of North Carolina at Chapel Hill who has written about Thai politics for decades.
The simmering conflict between the Bangkok-centered elites and the more rural-based populists has resulted in street protests, military coups, and violent clashes for nearly 15 years.
Ubolratana, a colourful, public-facing royal in contrast to her more restrained brother King Maha Vajiralongkorn, relinquished her royal titles after marrying the American Peter Jensen in 1972.
Commentators who on Friday assumed Ubolratana's bid had the blessing of her brother - the siblings are believed to be close - were at a loss for words following the king's rebuke.
"This makes Thai Raksa Chart, in a matter of seconds, the front-runner".
1975: She receives a master's degree in public health at the University of California, Los Angeles. Her full name is Ubolratana Rajakanya Sirivadhana Barnavadi.
Friday was the last day for parties to declare candidates.
"I would like to say once again that I want to see Thailand moving forward, being admirable and acceptable by worldwide countries, want to see all Thais have rights, a chance, good living, happiness to all", she said, concluding with "#ILoveYou".
Since then, she has acted in films and thrown herself into charity work, including a foundation to fight youth drug abuse. She is referred to as "Tunkramom Ying" meaning "Daughter to the Queen Regent".
