After leaving the royal family to follow her heart nearly 50 years ago, the 67-year-old sister of current monarch King Maha Vajiralongkorn is again upending protocol by entering politics.
Thailand has some of the most severe lese majeste laws in the world and the king's word is considered final. Though Ubolratana does not have any royaltitles 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.
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 party falls under the tutelage of Thaksin, who stands at the heart of Thailand's bitter political schism - loathed by the army and Bangkok elite, yet adored by the rural poor for health, welfare and education schemes. "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".
But Ubolratana's de facto alliance with the powerful political machine of exiled Thaksin - whose comeback the military has made every effort to block - puts Prayuth's supporters in an extremely awkward position. "I allowed Thai Raksa Chart Party to use my name for the nomination to the prime minister's post".
Ubolratana will run as a candidate for a party loyal to ousted premier Thaksin Shinawatra.
The candidacy would have broken with the tradition of the Thai royal family publicly staying out of politics.
The princess and the Thai Raksa Chart had not responded to the king's statement but the party did cancel a campaign event scheduled for Saturday.
Analysts said she is not technically covered by the kingdom's royal defamation law - which carries heavy jail sentences -but given the culture of deference to royalty, she is unlikely to face the scrutiny given to most politicians.
In 2016, Thais voted to approve a new constitution created by the country's military leaders, which was designed to perpetuate military influence and block Mr Thaksin's allies from winning another election.
The announcement effectively blocks Princess Ubolratana's unprecedented bid for the premiership and comes after an extraordinary rebuke of the candidacy by her younger brother, King Maha Vajiralongkorn.
Thai Raksa Chart Party chief Preechapol brushed aside questions about Thaksin. "I believe there will be no legal problems in terms of her qualification, but we have to wait for the Election Commission to endorse her candidacy", he said.
Ubolratana, a mathematics and biochemistry graduate from the Massachusetts Institute of Technology, has a reputation for being more accessible to commoners than the rest of the royal family.
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She returned to Thailand in the late 1990s after getting a divorce.
She was seen at the World Cup in Russian Federation smiling with Thaksin and his sister Yingluck - also a former prime minister - and has over the junta years given a number of coded nods on social media in favour of the Shinawatras. "The provisions also cover the queen, heir-apparent and royal family members close to the king".