The sister of Thailand's king has entered the race to become prime minister as the candidate of a populist party, an unprecedented foray into politics by a royal that instantly upended the first election since a 2014 military coup.
Princess Ubolratana Rajakanya Sirivadhana Barnavadi, 67, the elder sister of King Maha Vajiralongkorn, was nominated by 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 chief 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.
It was not immediately clear whether the nomination of Ubolratana, who has starred in Thai soap operas and lived in California for years, had the approval of King Vajiralongkorn.
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," Thitinan Pongsudhirak, a political analyst at Chulalongkorn University, told Reuters.
Ubolratana relinquished 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.
The conflict between the Bangkok-centred, royalist elites and Thaksin and his more rural-based supporters has resulted in street protests, military coups, and violent clashes over almost 15 years.
"Previous assumptions and scenarios have to be reconsidered," 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 off-shoot of the main pro-Thaksin Pheu Thai party whose government, led by Thaksin's sister Yingluck Shinawatra, was ousted from power in 2014 in a coup led by then-army chief Prayuth.
"The party has nominated the princess as its sole candidate," Thai Raksa Chart Party leader Preechapol Pongpanich told reporters at the Election Commission.
A small pro-military party, the People's Reform Party, asked the Election Commission to consider whether the princess's candidacy violated laws forbidding parties from invoking the monarchy in campaigns.
Ubolratana, the eldest child of King Bhumibol Adulyadej, 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.
Ubolratana is known for her "To Be Number One" anti-drugs program as well as starring in several soap operas and movies.
Her son was killed in the Indian Ocean tsunami in 2004.