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.
Former 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.
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 the opposition from Ms Ubolratana's younger brother, a constitutional monarch, is likely to lead to her disqualification by the Election Commission.
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 is considered extremely inappropriate," the King said in a statement issued by the palace.
The late-night statement was issued by the palace and later read on air by a television announcer.
King Maha 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 nomination of a member of the royal family by the pro-Thaksin Thai Raksa Chart Party could transform an election that had been viewed as a straightforward battle between Thaksin's populists and their allies, on the one hand, and the royalist-military establishment on the other.
"This is a profound development that will shape the contours and dynamics of Thai politics before and after the election ... Thai Raksa Chart is a leading contender now," Dr Thitinan Pongsudhirak, a political analyst at Chulalongkorn University, told Reuters.
Ms Ubolratana relinquished her royal titles in 1972 when she married an American, a fellow student at the Massachusetts Institute of Technology (MIT), Mr Peter Jensen. She lived in the United States for more than 26 years before they divorced in 1998.
Earlier on Friday, she 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.
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.
Thaksin and his party have been accused by their enemies of being opposed to the monarchy, which they have always rejected.
Pundits were earlier guessing whether Ms Ubolratana's nomination was a bid to heal those divisions, with the approval of the King who assumed the throne after the death of his father in 2016, or a bold gambit by Thaksin loyalists to undercut the royalist appeal of the pro-establishment parties.
"Previous assumptions and scenarios have to be reconsidered," Dr Thitinan said. "If this turns out well, it will be a process of reconciliation and unity. If this does not turn out well, it will pose alarming risk and even greater risk for Thailand's political future."
Ms Ubolratana's foray into politics follows a long period of mourning for King Bhumibol Adulyadej, who died in October 2016, and as her brother establishes himself on the throne.
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.
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.
Ms Ubolratana's candidacy was questioned by a small pro-military party, the People's Reform Party, which asked the Election Commission to consider whether it violated laws forbidding parties from invoking the monarchy in campaigns.
But Thai Raksa Chart Party leader Preechapol Pongpanich told reporters at the Election Commission he was not worried about challenges.
"She is knowledgeable and is highly suitable. I believe there will be no legal problems in terms of her qualification, but we have to wait for the Election Commission to endorse her candidacy," Mr Preechapol said.
The Election Commission is required to endorse or reject all candidates by next Friday (Feb 15).
Prime Minister Prayut Chan-o-cha, who was army chief when he led the 2014 coup and now heads the ruling junta, accepted his nomination from the Palang Pracharath 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.
Ms Ubolratana, the eldest child of King Bhumibol, was born in Lausanne 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.
Ms Ubolratana is known for championing an anti-drugs programme and has also starred in several soap operas and movies.
Her son was killed in the Indian Ocean tsunami in 2004.
On her Instagram account, Ms Ubolratana recently posted videos of her enjoying street food and complaining about Bangkok's pollution.
Soon after Friday's announcement, her catchphrase #SongPhraSlender (Long Live Slender) was number one on Twitter in Thailand.
"I've been bored with politics for so long, but the princess has made me happy about the election again," one fan wrote on a web forum.