She would have been pitted against the preferred candidate of the pro-royalist military, Prime Minister Prayuth Chan-ocha, who led the 2014 ouster of an elected government.
Observers immediately suggested the princess's candidacy was likely to unify the two-decade turmoil between the wealthy Shinawatras - triumphant in all elections since 2001 - and royalist urban elites who have backed the 2006 and 2014 coups against them. She lived in the US for more than 26 years before they divorced in 1998.
In the statement, the pro-Thaksin party also thanked Ubolratana for her "kindness toward the party" and vowed to push for policies that will bring prosperity for Thailand.
Because Thailand is a constitutional monarchy, the king and other royal family members are not supposed to get directly involved in politics.
Thai Raksa Chart responded swiftly, cancelling a campaign event on Saturday and issuing a statement saying it "complies with the royal command".
"Thank you for all the love and encouragement", the princess, who gave up her royal titles to marry a foreigner, said in a typically colourful Instagram post on Friday afternoon. The party is linked to former Thai leader Thaksin Shinawatra and his sister, Yingluck Shinawatra.
"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.
She lived in the United States for years, though frequently visited her parents, the much-revered King Bhumibol, who died in 2016, and Queen Sirikit. Aligning with a top royal could've potentially cleared the way for his return to Thailand, where he faces jail time for a conviction in a corruption case he has called politically motivated.
His Majesty made clear in the statement that although the princess had resigned from the royal family by law, she is still a royal family member by tradition.
"I am not aiming to extend my power but I am doing this for the benefit of the country and the people", he said. Hours later, she also spoke about how she was a commoner in Thailand as she had relinquished her title and wanted to help the common people of the country, reported Bloomberg.
After her marriage and 1973 graduation from MIT, she moved with her family to California and used the name Julie Jensen, earning a master's degree in public health at the University of California, Los Angeles.
Shortly after the king's statement she posted again without addressing the issue directly, simply thanking Thais for their support and saying that she wanted Thailand to "move forward and become admired and accepted by the global community".
Her son was killed in the Indian Ocean tsunami in 2004.
Soon after Friday's announcement, her catchphrase #SongPhraSlender (Long Live Slender) was number one on Twitter in Thailand.
2003: Ubolratana stars as a princess in a soap opera set in the Ayutthaya period, "Kasattriya".
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