The late-night statement from the palace left no doubt over the king’s displeasure at his sister’s move, laying bare an unprecedented public split in the royal family.

“Involvement of a high-ranking member of the royal family in politics, in whatever way, is an act that conflicts with the country’s traditions, customs and culture, and therefore is considered extremely inappropriate,” the king said in a statement issued by the palace.

The statement was later read on air by a television announcer.

King Vajiralongkorn also cited a provision in the constitution that states the monarch “exists in a status above politics” and maintains political neutrality.

“All royal family members adhere to the same principles … and cannot take any political office because it contradicts the intention of the constitution.”

The king’s statement did not directly criticise Ubolratana, instead praising her public work.

But it appeared to blame those linked to her sudden stride into politics.

Thai princess is election game-changer – but don’t write off junta yet

The princess, who gave up her royal titles to marry a foreigner, took to Instagram on Friday to say she was allowed to run under the constitution as a “commoner” had “allowed Thai Raksa Chart party to use” her name.

Friday was the last day for parties to declare candidates.

Prime Minister Prayuth Chan-ocha, who was army chief when he led the 2014 coup and now heads the ruling junta, also announced his candidacy on Friday.

Election authorities have a week to review which candidates put up by each party are allowed to run for premier after the March 24 poll.

Ubolratana’s candidacy had electrified the build-up to the election, which has long seemed poised to return the ruling junta and its proxies to power, in a straightforward battle against Thaksin’s populists and their allies.

Her involvement gave a royal sheen to the political machine of Thaksin, the self-exiled billionaire whose parties have won every election since 2001.

He 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.

The announcement appeared set to thrust Thaksin back onto the centre stage of Thailand’s political drama.

He was toppled in a 2006 coup, while his sister Yingluck Shinawatra was booted from power in a 2014 military takeover and forced into exile to avoid a jail term.

The king’s intervention has now cast Thaksin’s future role in politics into doubt.

Ubolratana, a popular and public-facing royal, relinquished her royal titles after marrying the American Peter Jensen in 1972.

But the couple divorced and she moved back to Thailand, where she is still considered part of the royal family.

In Thailand, she experienced tragedy, losing her autistic son Poom to the 2004 Asian tsunami.

Known to the public for lead roles in Thai films, onstage singing performances, a vibrant fashion sense and a sizeable Instagram following, Ubolratana is the first-born child of late king Bhumibol Adulyadej, who is still revered.

Her stride into politics by a royal left Thais scrambling to work out what it may mean for the nation’s tattered democracy.

“This is quite unprecedented and nobody is prepared for this,” Professor Anusorn Unno of Thammasat University said.

“I don’t think it’s the victory for the people, I think this is part of the adaptation of the ruling elite in terms of changing the landscape of politics.”

Since ascending the throne in 2016, Vajiralongkorn has reorganised palace affairs.

The vastly wealthy Crown Property Bureau is now under his personal stewardship, he has appointed several new privy counsellors and established a highly trained personal guard.

He also appointed a new army chief from a different faction of the military to the ruling junta.

His formal coronation will be held in May.