James Buchanan, another Thai politics watcher, wondered aloud whether the exiled former prime minister Thaksin Shinawatra had been left in the lurch by Ubolratana's candidacy.
Before the king's statement, Ubolratana had issued a statement on Instagram saying she has "no special privileges above the Thai people under the constitution".
Thailand became a constitutional monarchy in 1932 but the royal family wields great influence and commands the devotion of millions of Thais.
Ms Ubolratana's foray into politics follows a long period of mourning for King Bhumibol Adulyadej, who died in October 2016, and as her brother establishes himself on the throne.
"Bringing high-ranking royal family members to be involved in the political system, in any way, is an act that is against the ancient royal protocol and national custom and culture, and is seen as a highly inappropriate act", the statement added.
He said the latest development signalled a split in the royal family, which was "overtly intervening in Thai politics".
Surrogate parties that were patently Thaksin creations continued to win elections, the last one being Pheu Thai, headed by his sister Yingluck, which came to power in 2011 and was piloted by him from his bolt-hole in Dubai under the slogan "Thaksin thinks, Pheu Thai Acts".
The election, scheduled for March 24, was widely expected to leave the country in the hands of the junta even though opposition parties were believed capable of winning a plurality in the lower house of the parliament, despite an electorate largely fed up with the military's stewardship of the country and by ostentatious displays of corruption on the part of the generals.
However, the nomination of a member of the royal family by the pro-Thaksin Thai Raksa Chart party could change that dynamic.
Thai Princess Ubolratana Rajakanya is running to be her country's next prime minister, throwing Thailand's political scene into a new state of uncertainty ahead of elections next month.
"I would like to say once again that I want to see Thailand moving forward, being admirable and acceptable by global countries, want to see all Thais have rights, a chance, good living, happiness to all", she said, concluding with "#ILoveYou".
Ms Ubolratana, the eldest child of King Bhumibol, was born in Lausanne in 1951.
The main opponent of the princess, if her nomination were to stand, would be Prayuth, who also announced his candidacy on Friday, as the candidate for a new pro-military party, set up by his loyalists. She lived in the US for more than 26 years before they divorced in 1998.
The Thai Raksa Chart party dropped the political bombshell this morning. In a sign of the extreme sensitivity, three Thailand-based legal and political analysts declined to comment on her candidacy.
Rivalry between the Bangkok-centred, royalist elites and Thaksin and his rural-based supporters has brought street protests, military coups, and violent clashes over nearly 15 years.
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.
"Friday's events were astounding and have people completely rethinking their assessments and perspectives and the country's future political trajectory", said Jay Harriman, senior director at BowerGroupAsia, a government affairs consultancy.
Thaksin's well-funded political machine returned to power twice since and is seen as the key element in arranging for Ubolratana's selection because she cast her lot with a Thaksin-affiliated party.
The monarchy is revered in Thailand, and is protected by some of the harshest lese majeste laws in the world. 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.
She's starred in several soap operas and movies, she's known for her charity work with young Thais, and is a prominent figure on social media.
Earlier on Friday, she thanked her supporters in an Instagram post. One of her children died in the 2004 Indian Ocean quake and tsunami the day after Christmas that killed hundreds of thousands.