The late-night statement was issued by the palace and later read on air by a television announcer.
The announcement pits a royal-fronted party tied to the Shinawatras directly against a military party, whose own candidate was also announced Friday as junta leader Prayut Chan-O-Cha.
Thai Raksa Chart was created a year ago as part of a strategy to diversify the clan's political assets in case its main vehicle, the Puea Thai party, was dissolved by the junta that deposed the government Yingluck helmed from 2011 to 2014.
She relinquished her royal titles in the 70s when she married an American.
She is regularly featured in the daily TV coverage of the royal family going about their duties in the Kingdom and is generally treated with the same respect as the other leading members of the family. Aligning with a top royal could've potentially cleared the way for his return to Thailand, where he faces jail time for a conviction in a corruption case he has called politically motivated. "In the event Thaksin-aligned parties win the election, it makes it very hard for the military and royalists to contest, protest, or seek to overturn the result", Hicken said. "I allowed Thai Raksa Chart Party to use my name for the nomination to the prime minister's post".
Indeed, May 2014 coup and repeated election postponements up to this year are a symptoms of a longer-term struggle that dates back to 2001, when the populist forces that brought then-prime minister Thaksin Shinawatra to power were at odds with the older elite establishment comprising the military, monarchy, and bureaucracy.
Thailand's chaotic politics took two astonishing turns Friday when the sister of the king made a historic bid to become prime minister, only to have him shut down her effort as "inappropriate" because it violated tradition and the constitution, both of which keep the monarchy from getting involved in politics.
The princess and the Thai Raksa Chart had not responded to the king's statement but the party did cancel a campaign event scheduled for Saturday.
Prayut, in sharp contrast, is a domineering, splenetic character - an autocratic former general who since he seized power has repressed the naturally rambunctious instincts of Thai politics and society in the self-serving cause of stability.
The simmering conflict between those two sides has resulted in street protests, military coups, and violent clashes for nearly 15 years.
Thai Raksa Chart shocked the country when it announced Ubolratana as its candidate on Friday morning.
Ubolratana's brother, King Vajiralongkorn, does not support her bid for prime minister, calling it "inappropriate" in a televised address.
But Thai Raksa Chart Party leader Preechapol Pongpanich told reporters at the Election Commission he was not anxious about challenges.
An Election Commission official said it is reviewing all candidate applications and will finish the process this week, though few expect it to divert from the royal order. She studied mathematics and biochemistry at the Massachusetts Institute of Technology (MIT) and earned a master's degree in public health from the University of California at Los Angeles.
She was seen at the 2018 World Cup in Russian Federation smiling with Thaksin and Yingluck and has over the junta years given a number of coded nods on social media in favour of the Shinawatras.
Thai Prime Minister Prayuth Chan-ocha is loyal to the junta.
"The party has nominated the princess as its sole candidate", Thai Raksa Chart Party leader Preechapol Pongpanich told reporters at the Election Commission.
Princess Ubolratana, the oldest daughter of King Bhumibol, was born in Lausanne in 1951.
"I would like to say once again that I want to see Thailand moving forward, being admirable and acceptable by worldwide countries, want to see all Thais have rights, a chance, good living, happiness to all", she said, concluding with "#ILoveYou".