Thai Princess Ubolratana Rajakanya Sirivadhana Barnavadi, seen here in May 18, 2009, planned to run for prime minister but not a seat in parliament, which is allowed under the election law.
The Thai Election Commission has until next Friday, Feb. 15, to accept or reject the princess's candidacy for the March 24 elections.
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.
Thailand has been a constitutional monarchy since 1932.
The sudden entry today (Feb.8) of Princess Ubolratana Rajakanya Sirivadhana Barnavadi, the 67-year-old elder sister of King Vajiralongkorn, into Thai politics, with the announcement that she would head the Thai Raksa Chart Party, is nearly certain to bring Thaksin Shinawatra, the country's wiliest politician, back to the center of power, to the utter dismay of the military which has run the country since a 2014 coup.
But the statement late Friday from the palace condemned the move "to bring" Ubolratana into politics as "highly inappropriate" and "unconstitutional", dimming the prospects of her running.
But the monarchy could now be seen as effectively aligned with Thaksin's populist political movement, and if Ubolratana is to become prime minister, it could also pave a path forward for the return of the Thaksin family to Thailand.
It said that in all constitutions - including the current - "the king stays above politics", which also means the queen, the heir apparent and the king's immediate family must remain "politically neutral" and "cannot hold any political positions, as it's a violation of the constitution and the tradition of constitutional monarchy".
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On Friday morning, it was confirmation of Ubolratana's unprecedented candidacy that was the stunner, as the development grabbed headlines and saw footage of the 67-year-old, in recent times an actress, aired repeatedly on global news channels.
The long-delayed election will be the first since former army chief Prayuth Chan-Ocha seized power in a coup in 2014 after a period of unrest, becoming the leader of the country's military government.
FILE - Thailand's Princesses Ubolratana Rajakanya attends a procession to transfer the royal relics and ashes of Thailand's late King Bhumibol Adulyadej from the crematorium to the Grand Palace in Bangkok, Oct. 27, 2017.
"Who would dare criticise a royal prime minister?" said Puangthong Pawakapan, professor of political science at Chulalongkorn University. She reacquired the title upon her divorce with husband Peter Ladd Jensen in 1998.
Thai Raksa Chart Party chief Preechapol brushed aside questions about Thaksin. She had to give up her highest royal titles more than 40 years ago when she married an American man and moved to the United States.
The simmering conflict between the Bangkok-centered elites and the more rural-based populists has resulted in street protests, military coups, and violent clashes for nearly 15 years.
Thaksin is a Thai businessman and former politician who was prime minister from 2001 to 2006.
The BBC's Jonathan Head said the move to nominate the princess looks like a grave miscalculation, one likely to weaken the pro-Shinawatra faction trying to push the military out of politics. But she is still regarded in the public mind as a member of the royal family, and her entry into politics sets an unprecedented quandary for her opponents in the March elections.
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"I think that it's part of a plan by the current sovereign to increase his personal power across the country".
She had earlier on Friday thanked her supporters in an Instagram post.
Princess Ubolratana Mahidol's nomination by the opposition Thai Raksa Chart Party marks a shock realignment of Thai politics.
She's starred in several soap operas and movies, she's known for her charity work with young Thais, and is a prominent figure on social media.
On her Instagram account, Ms Ubolratana recently posted videos of her enjoying street food and complaining about Bangkok's pollution.
Her son was killed in the Indian Ocean tsunami in 2004.
Ubolratana's decision to link up with a Thaksin-backed party raised the possibility that she could help push through changes to the Constitution that would make it more democratic.
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