Thailand's King Maha Vajiralongkorn has said it is inappropriate for his sister to enter the race to become prime minister in next month's election.
A Royal Gazette statement issued late on Friday said Princess Ubolratana Rajakanya’s candidacy is against royal tradition and the constitution The king added that being part of the royal family requires remaining apolitical.
“Bringing high-ranking members of the royal family into politics- in any manner- is against the royal traditions and is gravely inappropriate,” the king said. “The Thai monarchy is the centre that glues the hearts of the Thai people together. The monarch and the royal family members are above politics.”
Princess Ubolratana announced her surprise candidacy for a populist party on Friday, an unprecedented foray into politics by a royal that instantly upended the first election since a 2014 military coup.
The 67-year-old, the elder sister of King Vajiralongkorn, will run as a candidate for a party loyal to ousted premier Thaksin Shinawatra, the figure at the centre of political turbulence and rival street protests that have riven Thai society for years.
One of her leading opponents in the March 24 election will be Prime Minister Prayuth Chan-ocha, the leader of the ruling military junta, who also announced his candidacy on Friday.
Thailand has been a constitutional monarchy since 1932 but the royal family has wielded great influence and commands the devotion of millions.
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," said Thitinan Pongsudhirak, a political analyst at Chulalongkorn University.
Princess Ubolratana relinquished her royal titles in 1972 when she married an American, a fellow student at the Massachusetts Institute of Technology, Peter Jensen. She lived in the United States for more than 26 years before they divorced in 1998.
The simmering conflict between the Bangkok-centred, royalist elites and Mr Thaksin and his more rural-based supporters has resulted in street protests, military coups, and violent clashes over almost 15 years.
Pundits were left guessing whether the princess's nomination was a bid to unify those divisions, with the approval of the king, who assumed the throne after the death of his father in 2016, or a bold move by Thaksin loyalists to undercut the royalist appeal of the pro-establishment parties.
"Previous assumptions and scenarios have to be reconsidered," Mr 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."
Thai Raksa Chart is an offshoot of the main pro-Thaksin Pheu Thai Party whose government, led by Mr Thaksin's sister Yingluck Shinawatra, was ousted from power in 2014 in a coup led by then-army chief Prayuth.
Mr Thaksin and his party have at times been accused by enemies of being opposed to the monarchy. a claim they have always rejected.
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.
"The party has nominated the princess as its sole candidate," Thai Raksa Chart Party leader Preechapol Pongpanich told reporters after registering his party's candidate at the Election Commission.
Princess Ubolratana is running for prime minister but not a seat in parliament, which is allowed under the election law.
"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," he said.
The Election Commission is required to endorse or reject all candidates by next Friday.
Mr 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.
There was no mention of the princess's nomination in Mr Prayuth's statement.
Princess Ubolratana, the oldest daughter 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.
But she would not be covered by Thailand's strict lese majeste law against insulting the monarchy.
Princess Ubolratana is known for her "To be Number One" philanthropy campaign, which aims to help young people stay away from drugs, as well as starring in several soap operas and movies.
Her son was killed in the Indian Ocean tsunami in 2004.
On her Instagram account, the princess recently posted a video of her eating street food and another complaining about Bangkok's pollution.
Soon after Friday's announcement, her catchphrase #SongPhraSlender (Long Live Slender) was number one on Twitter in Thailand.
"I have been bored with politics for so long, but the princess has made me happy about the election again," one fan wrote on the web forum Pantip.