Thailand's King Maha Vajiralongkorn said his elder sister's announcement she is running for prime minister in March elections is "inappropriate" and unconstitutional, likely sinking her candidacy for a populist opposition party.
Paiboon Nititawan, the leader of the People's Reform Party, handed a letter to the Election Commission calling on it to decide whether to invalidate the nomination of Princess Ubolratana Rajakanya Sirivadhana Barnavadi.
But while Ubolratana is afforded the same privileges, she has cultivated a more accessible image than her reserved younger brother and shown a knack for reading the sentiments of ordinary Thais.
A Thai political party on Friday named a princess as its nominee to be the next prime minister, upending tradition of the palace eschewing politics and upsetting all predictions about what might happen in next month's elections.
The king also cited a provision in the constitution that states the monarch stays above politics and maintains political neutrality.
The development raises questions about the future of other political candidates, amid speculation that no-one will run against a member of the royal family.
Thailand is now led by the military, which ousted Thaksin in a coup in 2006 and later helped remove his sister, Yingluck Shinawatra, from the role of prime minister in 2014.
"Royalists who had been appalled by the prospect of a political deal with Thaksin are now rejoicing, while Thais who believed the deal heralded an end to political conflict are now heartbroken", he said in a statement.
"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.
In this March 24, 2010, photo, Thai Princess Ubolratana poses for a photo during her visit to promote Thailand's film industry at the Entertainment Expo Hong Kong Filmart. What actually had happened behind the scenes is unlikely to become public, because the Thai royal family's private affairs are nearly never leaked.
"There are unsafe days ahead", he said.
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Political analysts see the shocking move as a further political complication in the South-East Asian country ahead of its first election since the 2014 coup and return to democracy.
"I am deeply touched", Ubolratana posted on Instagram.
Thai Raksa Chart is an offshoot of Pheu Thai, the latest incarnation of Thaksin's party that has won every election since 2001. 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.
"It'll be hard for parties to run against the princess", said Paul Chambers, a lecturer at Naresuan University's College of ASEAN Community Studies in the country's north. "Voters would find it hard to choose someone that's not part of her party, because Thai ideology puts the royals at the top".
After marrying in 1972, the Princess relinquished her royal title and moved to the US. She has three children, one of whom died in the 2004 tsunami that killed more than 225,000 people in southern Asia.
"Just a theory: Thaksin creates Thai Raksa Chart to mitigate the risk of Puea Thai being dissolved".
"She supported Thaksin and wanted to participate in politics", said Paul Chambers, a lecturer at Naresuan University.
On her Instagram account, the princess recently posted a video of her eating street food and another complaining about Bangkok's pollution.
Like all the parties contesting the election, it had to submit its candidate list to the Election Commission on Friday. It was not immediately clear if Ubolratana would be covered by the kingdom's hardline royal defamation law - which carries up to 15 years in jail per charge - or how that could impact debate and criticism in the run up to elections.
Even for a country where coup attempts, street protests and political intrigue are the norm, Feb 8 will go down as one of the most dramatic days in Thai history.