The Thai Election Commission has until next Friday, Feb. 15, to accept or reject the princess's candidacy for the March 24 elections.

The older sister of the king of Thailand said Friday she will run for prime minister in elections next month, upending the palace's decades-long tradition of eschewing politics and setting up a surprise contest with the leading military-backed candidate.

It is being closely watched as the first chance for Thailand to return to democracy after five years under military rule. Ubolratana, 67, was born into royalty but is not exactly a royal princess, which distinguishes her from her three siblings: Vajiralongkorn, 66, Princess Sirindhorn, 63, and Princess Chulabhorn, 61.

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.

The statement cited a passage of the constitution that says the monarchy should maintain political neutrality.

A Thai political party will obey a command from the king blocking the candidacy of a princess for prime minister, it said in a statement Saturday, in a dramatic reversal only a day after putting her forward for the position.

Yet a small party that supports the junta later Friday submitted a letter to the Election Commission objecting to the nomination of the princess, saying it should be suspended as it could violate election law.

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.

The nomination of a member of the royal family by the pro-Thaksin Thai Raksa Chart Party could transform an election that had been viewed as a straightforward battle between Thaksin's populists and their allies, on the one hand, and the royalist-military establishment on the other.

Known to the public for lead roles in Thai films, onstage singing performances, a vibrant fashion sense and a sizeable Instagram following, Ubolratana is the first-born child of the late King Bhumibol Adulyadej.

Repeated coups have also pushed it away from its traditional alliance with the United States and more towards China - a move many politicians standing in the election oppose.

Thailand's Prime Minister Prayut Chan-o-cha later accepted the nomination. She lived in the US for more than 26 years before they divorced in 1998.

The Thai Raksa Chart Party was launched in November by relatives and supporters of the Shinawatras.

After marrying in 1972, the Princess relinquished her royal title and moved to the US. Violent street protests and two military coups have marked the years since.

It may well be that Ubolratana, a former actress, singer and activist, was encouraged to enter politics by her brother, who is using her as a weapon in consolidating his formidable power in the country, but it is nearly certain that Thaksin played a major role in the decision.

The announcement thrusts him back to the centre stage of Thailand's political drama just as it appeared the military were set to succeed in sidelining him. Though Ubolratana does not have any royal titles and is not covered under lese majeste laws as of now - she relinquished her royal status back in 1972 - it is hard to imagine that her status would not affect how polls would be conducted.

"I think that it's part of a plan by the current sovereign to increase his personal power across the country".

But after their divorce she returned home to Thailand, where officials treat her as a member of the royal family.

The princess has a heavier media presence than any of her siblings, ranging from appearances in Thai movies and television to an Instagram page with about 100,000 followers.

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.

Tragedy struck in 2004 when her autistic 21-year-old son Bhumi died in the Indian Ocean tsunami.

The move by King Maha Vajiralongkorn's sister shocked a nation where top royals are officially treated with semi-divine status and protected by strict lese majeste laws that shield them from criticism.

Ubolratana made her first trip back to her homeland in 1980 for one of her mother's birthdays.