On Friday, one of the parties supporting Prayut had filed a petition opposing Ubolratana's candidacy, arguing it could breach a law that prohibits the use of the monarchy in campaigning.
But her foray into politics - breaking with royal tradition - looked to be short-lived after her younger brother, King Maha Vajiralongkorn, quickly signalled he opposed it, which is likely to lead to her disqualification.
Despite being a a constitutional monarchy since 1932, Thailand's royal family has wielded great influence and commanded the devotion of millions.
Princess Ubolratana Mahidol, older sister of Thailand's King Maha Vajiralongkorn, is now a candidate for prime minister - a move that marks a startling break with Thai royalty's long-observed practice of staying out of politics.
"Bringing a high-ranking member of the royal family to politics, in whatever manner, is an act in violation of the royal tradition and national culture and highly inappropriate", the announcement reads.
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. Provided that nomination still holds and continues on to the election date, Ubolratana's chief opponent will be Prayut, who also confirmed his expected candidacy for the Palang Pracharat Party set up by loyalists.
The nomination of a royal family member by pro-Thaksin forces was an audacious gambit, potentially undercutting Thaksin's ardently royalist foes, and setting up an election showdown with Prime Minister Prayuth Chan-ocha, who led the 2014 coup and heads the military government.
Thailand's King Vajiralongkorn has denounced as "inappropriate" his sister's unprecedented bid to run for prime minister in March's election.
"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.
Princess Ubolratana is the eldest child of King Bhumibol Adulyadej, Rama IX, and Queen Sirikit.
Ole Gunnar Solskjaer has shared Manchester United vision with Ed Woodward
The Gunners will face Belarusian side BATE Borisov in the last 32 of the competition starting with the first leg next week.
The King's intervention has cast Thaksin's future role in politics into doubt.
And it means next month's election may prove a genuine contest, not merely a joyless coronation of Prayut Chan-o-cha, leader of the military junta. They settled in the United States where she and her husband, Peter Jensen, had three children.
"This is a senior royal member, the sister of the king, so this is unprecedented in Thai politics", said Thitinan Pongsudhirak, director of the Institute of Security and International Studies at Chulalongkorn University in Bangkok. The princess lived in the US for years, but after the couple divorced in 1998, she then moved to Thailand, where she is still referred to as a princess and a member of the royal family.
The conflict between the Bangkok-centered, royalist elites and Thaksin and his more rural-based supporters has resulted in street protests, military coups, and violent clashes over nearly 15 years.
"Assuming that this is proceeding with the blessing of the king, this suggests that the crown is trying to chart its own course through Thailand's turbulent political waters, placing some distance between itself and the military", Hicken said.
"I am not aiming to extend my power but I am doing this for the benefit of the country and the people", he said. But she posted photos on her Instagram page saying she was in the northern city of Chiang Mai, along with the cryptic message "we will walk together". In general, like most of the royal family, she publicly kept herself aloof from Thailand's recent political turmoil.
Ubolratana is known for her "To Be Number One" anti-drugs program as well as starring in several soap operas and movies.
The 67-year-old princess was nominated Friday by the opposition Thai Raksa Chart Party. One of her children died in the 2004 Indian Ocean natural disaster and tsunami the day after Christmas that killed hundreds of thousands.
Thaksin, a telecommunications billionaire, remains wildly popular outside Thailand's cities, where he offered cheap medical care and debt relief.
If she does remain as a candidate, there are other question that remain, chief among them being how the competitive dynamics of the election would be affected.