King Vajiralongkorn, 66, issued his message late on Friday, saying his elder sister's candidacy was "inappropriate" and it was against the spirit of the constitution for royalty to be involved in politics.
A new Thai political party vowed Saturday, February 9, to obey a command from the king blocking the candidacy of a princess for prime minister in a dramatic reversal that appeared to boost the junta's chances ahead of March elections.
He is a former army chief who led the 2014 military coup that ousted the country's last elected government, led by Yingluck Shinawatra.
For Thaksin, it amounts to another setback - one of many since he was first removed in a 2006 coup, in part due to accusations of undermining the monarchy.
Both Mr Thaksin and his sister live in self-imposed exile but remain a powerful force in Thai politics, with many in the country remaining loyal to them.
Traditionally the Thai royal family is known to publicly stay out of the country's politics.
The palace statement did not directly criticise the princess but seemed to take aim at the political party bringing her on board, a move described as "against royal traditions and the nation's culture".
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The princess accepted the nomination of the Thai Raksa Chart party - one of several parties linked to Thaksin and his sister Yingluck - meaning, the candidacy would at least have needed his endorsement.
Thai Princess Ubolratana Rajakanya Sirivadhana Barnavadi, seen here in May 18, 2009, planned to run for prime minister but not a seat in parliament, which is allowed under the election law.
"Even though she relinquished her title according to royal laws ... she still retains her status and position as a member of the Chakri dynasty", the King's order said. She said she "sincerely wished" to see the kingdom progress and for people to enjoy unfettered rights, opportunities and happiness. 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. "It is not easy for me to make my decision.it is a crucial moment for the country", he said.
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.
She thanked supporters on Saturday on her widely followed Instagram account, saying vaguely that she wanted Thailand to "move forward". She is referred to as "Tunkramom Ying", which means "Daughter to the Queen Regent", and is treated by officials as a member of the royal family. While it does not technically apply to Ubolratana, who lost her highest royal titles when she married an American more than four decades ago, its scope has been widened in recent years to nearly anything that sullies the royal institution, making criticism of the princess highly problematical.
Her son was killed in the Indian Ocean tsunami in 2004. The other two now also live in Thailand.
However, the royal family is revered in Thailand and rarely criticised, so there are questions around whether any other candidate would want to challenge a member of the royal family.