The King of Thailand has blocked his sister's application to become prime minister.
In a late-night order, King Maha Vajiralongkorn said that although Princess Ubolratana Mahidol had given up her title in 1972, "she retains her status and position as a member of the Chakri dynasty."
For them, it would be a "most inappropriate act," he added.
In an Instagram message on Saturday, the princess, who had starred in several soap operas and films, did not mention the king's order, but thanked the people for their support.
It was announced on Friday that she would run as a candidate for the Thai Raksa Chart party, which is loyal to ousted Prime Minister Thaksin Shinawatra, whose ally has won all national elections since 2001.
Thai Raksa Chart attempts to defeat Prayuth Chan-ocha, leader of the Thai military junta, in the March 24 election.
It will be the first choice since a pro-royalist military coup in 2014.
The Thai Raksa card has now sworn allegiance to the king. She said she had accepted his order and also thanked the princess.
Earlier, party leader Preechapol Pongpanich said the princess was "knowledgeable and highly capable," adding that he believed that "there were no legal issues regarding their qualifications."
The king said in his order, "Although she gave up her title under royal laws, she retains her status and position as a member of the Chakri dynasty.
"The incorporation of high-ranking members of the royal family in any way into the political system is an act that violates the old royal protocol and the national custom and culture and is considered to be extremely inappropriate."
Thaksin Shinawatra continues to be popular with the rural majority in Thailand, who loved policies such as general health care and rice subsidies.
However, his popularity was viewed with suspicion by the Thai establishment, which saw her as a threat to the monarchy.
Born in Switzerland, Princess Ubolratana studied mathematics and biochemistry at the Massachusetts Institute of Technology and received a master's degree in public health from the University of California.
She gave up her royal title in 1972 after she married an American student. They lived in the US for more than 26 years, but divorced in 1998.
She returned to Thailand in 2001 and continued to perform royal duties, but never regained her full title.