Bangkok: Thailand's King Maha Vajiralongkorn on Friday moved to block his his elder sister's surprise bid to run for prime minister in March, calling her candidacy for a populist opposition party "inappropriate" and unconstitutional.
Princess Ubolratana Rajakanya Sirivadhana Barnavadi, 67, stunned the nation when she announced on Friday she would be the sole prime ministerial candidate for the party, which is loyal to ousted ex-premier Thaksin Shinawatra, in the March election.
Her candidacy instantly threatened to upend the first national ballot since a military coup in 2014 that ousted a government loyal to Thaksin, the figure at the centre of years of political turbulence and rival street protests that have riven Thai society.
But her foray into politics looked to be short-lived after the public opposition from King Vajiralongkorn, which is likely to lead to the Election Commission disqualifying her or the princess dropping out of the race.
Thailand has been a constitutional monarchy since 1932, but the royal family has wielded great influence and commands the devotion of millions.
"Involvement of a high-ranking member of the royal family in politics, in whatever way, is an act that conflicts with the country's traditions, customs, and culture, and therefore considered highly inappropriate," the king said in a statement.
The statement was issued by the palace and later read on air by a television announcer.
King Vajiralongkorn also cited a provision in the constitution that states the monarch stays above politics and maintains political neutrality.
"All royal family members adhere to the same principles ... and cannot take any political office, because it contradicts the intention of the constitution."
Friday was the last day for parties to declare candidates.
The Election Commission declined to comment when contacted by Reuters late on Friday night, with an official saying its members will hold a meeting on Monday.
The princess's nominating party, Thai Raksa Chart - an offshoot of the larger pro-Thaksin party that was ousted from power in the 2014 coup - could not be reached for comment.
Prime Minister Prayuth Chan-ocha, who was army chief when he led the 2014 coup and now heads the ruling junta, also announced his candidacy on Friday.
Ubolratana, who has starred in Thai soap operas and a movie, relinquished most of her royal titles in 1972 when she married an American, a fellow student at the Massachusetts Institute of Technology (MIT), Peter Jensen.
She lived in the United States for more than 26 years before they divorced in 1998.
She had earlier on Friday thanked her supporters in an Instagram post.
"I have accepted the Thai Raksa Chart Party nomination for prime minister to show my rights and freedom without any privileges above other fellow Thai citizens under the constitution," she said.
Her Instagram account was silent immediately after the king's statement.
Nominating a member of the royal family had seemed a potential game-changer for the Thaksin loyalist parties that have been accused by their enemies of being opposed to the monarchy, charges they have always rejected.
Rivalry between the Bangkok-centred, royalist elites and Thaksin and his rural-based supporters has brought street protests, military coups, and violent clashes over almost 15 years.
Ubolratana's announcement followed a long period of mourning for King Bhumibol, who died in October 2016, and as her brother establishes himself on the throne in preparation for an official coronation in May.
Thai Raksa Chart is an off-shoot of the main pro-Thaksin Pheu Thai party whose government, led by Thaksin's sister Yingluck Shinawatra, was ousted in the 2014 coup. Thaksin himself was overthrown in a 2006 coup and both he and Yingluck now live in exile.
Pheu Thai is also fielding candidates in the election, and Thai Raksa Chart was formed by Thaksin loyalists and the core leadership of the United Front for Democracy Against Dictatorship (UDD), or "red shirts" group, as a strategy to help Pheu Thai win seats, or to act as a back-up if the main party was disqualified.
Junta leader Prayuth accepted his nomination from the Palang Pracharat Party, a new party set up by his loyalists, in an official statement.
"I am not aiming to extend my power but I am doing this for the benefit of the country and the people," he said.
Ubolratana, the eldest child of King Bhumibol Adulyadej, was born in Switzerland in 1951. She studied mathematics and bio-chemistry at MIT and earned a master's degree in public health from the University of California at Los Angeles.
She returned permanently to Thailand in 2001, performing royal duties but never regaining her full royal titles. She is referred to as "Tunkramom Ying", which means "Daughter to the Queen Regent", and is treated by officials as a member of the royal family.
Her 21-year-old son was killed in the Indian Ocean tsunami in 2004 while vacationing on a Thai island.