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.
Key events in Ubolratana's life:
1951: Ubolratana is born in Lausanne, Switzerland, the first child of King Bhumibol Adulyadej and Queen Sirikit while her father was studying science at the University of Lausanne. She is given the title Her Royal Highness, Chao Fa (lady of the sky) before the family moves back to Thailand in December that year.
1952: Ubolratana's brother, the future King Maha Vajiralongkorn, is born.
1960s: In her teenage years, she shared her father's love of sports and became his favourite partner in tennis, badminton and small-boat sailing. The pair shared a gold medal in sailing at the Southeast Asian Games in 1967.
1969: The princess sets off for Boston to study nuclear physics at the Massachusetts Institute of Technology (MIT).
1972: She meets and marries American Peter Jensen, an MIT classmate. She relinquishes her royal titles as required by marrying a foreigner.
1973: Ubolratana graduates from MIT with a bachelor's degree. She later settles in southern California taking the name Julie Jensen. She has three children with Jensen and does not return to Thailand for eight years.
1975: She receives a master's degree in public health at the University of California, Los Angeles.
1990s: Ubolratana makes more frequent trips to Bangkok, joining her mother, Queen Sirikit, at charity events and balls.
1991: Ubolratana appears more frequently in the limelight on Thai magazine covers, society parties and fashion events. She does not recover her full titles but is treated as a full princess, with staff prostrating themselves at her feet.
1992: She founds the Ubolratana Foundation under patronage of the Queen, which supports children orphaned by HIV-related illnesses.
1998: Ubolratana and Jensen divorce and she brings their three children back to Thailand and resumes royal duties. The children receive Thai citizenship and are treated as royals.
2001: Ubolratana and her children permanently return to Thailand. She is referred to as "Tunkramom Ying" meaning "Daughter to the Queen Regent".
2002: Ubolratana launches "To Be Number One" under royal patronage. The initiative is anti-drug campaign for at-risk youth and teens. She chairs the foundation, occasionally singing and dancing at awareness-raising programmes.
2003: Ubolratana stars as a princess in a soap opera set in the Ayutthaya period, "Kasattriya".
2004: Ubolratana's son, Bhumi, is killed in the Indian Ocean tsunami.
2008: She makes silver screen debut with "Where the Miracle Happens", a film based on a novel she wrote in which she stars as a businesswoman who becomes a volunteer teacher.
2016: King Bhumibol Adulyadej dies in October and her brother assumes the throne later in the year. Ubolratana and Thaksin's daughter exchange messages of support on social media during the mourning period for the late king.
2018: Pictures appear on social media of Ubolratana, Thaksin and his sister, former prime minister Yingluck Shinawatra, at the World Cup in Russia.
Feb. 8, 2019: A Thaksin-linked party nominates Ubolratana as candidate for prime minister for a general election set for March 24.
