Thai Raksa Chart party leader Preechapol Pongpanich, holds up application of candidate for Prime Minister, Thailand's Princess Ubolratana Rajakanya Sirivadhana Barnavadi, at the election commission office in Bangkok, Thailand February 8, 2019.
The statement was issued by the royal palace and later read on air by a television announcer.
News agency AFP noted in a profile of Ubolratana on Friday that her post accepting Thai Raksa Chart's nomination for prime minister candidate featured her wearing a traditional dress from that region.
The party thanked the 67-year-old princess for her efforts, and said it would abide by election regulations and "tradition and royal customs" under Thailand's constitutional monarchy. Prior to the King's remarks on Friday night, Ubolratana emphasised that she was a commoner after relinquishing her royal titles, and said she wanted "the opportunity to bring glory to the country".
Friday was the last day for political parties to submit their lists of prime minister candidates to the Election Commission.
Thailand has some of the world's toughest royal defamation "lese-majeste" laws but technically the princess is not covered by them. "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".
It remains possible Prayut's backers could mount a legal challenge to Ubolratana's candidacy by claiming she is violating laws created to keep the monarchy out of politics.
Others expressed unhappiness with groups such as Thai Raksa Chart linked to Yingluck and her brother Thaksin, who was ousted in a 2006 coup.
Thaksin's well-funded political machine returned to power twice since and is seen as the key element in arranging for Ubolratana's selection because she cast her lot with a Thaksin-affiliated party.
A princess wants to be the next prime minister of Thailand - an unprecedented move in a country where royals traditionally stay out of politics.
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"We are in new territory", said Kevin Hewison, an emeritus professor at the University of North Carolina at Chapel Hill who has written about Thai politics for decades.
Ubolratana defended her decision to run for office by saying she had relinquished all her royal titles and now lived as a common citizen.
Rivalry between the Bangkok-centred, royalist elites and Thaksin and his rural-based supporters has brought street protests, military coups, and violent clashes over nearly 15 years.
Most observers of Thai politics agree Thaksin aggressively pursued good relations with Ubolratana's brother, current King Maha Vajiralongkorn, and friendship with the princess herself.
"Thai Raksa Chart party complies with the royal command", it said.
Thai Raksa Chart is an offshoot of Pheu Thai, the latest incarnation of Thaksin's party that has won every election since 2001.
As of early Saturday morning, she had not withdrawn her candidacy, nor had Thailand's Election Commission officially disqualified her. 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. In terms of the outcome, while Prayut was seen as being the likely victor by far in upcoming polls, the most likely scenario now is a unity, royal-led government with Ubolratana at the helm, with Thaksin's influence at play and some role for Prayut-linked forces as well.
She is also well known and loved in Thailand as a keen singer and actor, appearing in several films, and is active on social media, where posts of her singing and dancing have gone viral on her Instagram page.
The siblings are the children of late King Bhumibol Adulyadej, who died in 2016 after a record breaking 70 years on the throne. Ubolratana's win would certainly indicate a rising role for the monarchy in the country's politics as she would be the first royal to win the premiership since Thailand moved away from absolute monarchy. "The provisions also cover the queen, heir-apparent and royal family members close to the king".