A Thai party close to ousted Prime Minister Thaksin Shinawatra on Friday nominated the elder sister of the country's king as a prime ministerial candidate for the upcoming general election that will end over four years of military rule.

However, King Maha Vajiralongkorn issued a palace statement expressing his opposition to Princess Ubolratnana Rajakanya's bid to lead the Thai Raksa Chart Party as a prime ministerial candidate.

The statement noted that the princess should stay away from politics as her involvement would constitute an act that defies the Thai royal family's customs and traditions.

It is extremely rare for Thai royalty to get involved in politics, but Princess Ubolratana Rajakanya, 67, reportedly has close ties with Thaksin, who lives in self-exile abroad.

Her foray into politics as the likely "face" of pro-Thaksin forces injects considerable uncertainty into the March 24 election that will involve several populist pro-Thaksin parties, the pro-junta Palang Pracharath Party and the conservative, royalist Democrat Party.

After filing the princess' candidacy with the Election Commission, the Thai Raksa Chart issued a statement saying she agreed to become its candidate as she wants to promote reconciliation in the country that has experienced intense political turmoil.

The princess also wants to help develop Thailand with her experiences both in Thailand and overseas, according to the statement.

The Thai Raksa Chart was founded by former parliamentarians from the Pheu Thai Party, widely perceived as Thaksin's political vehicle.

Pheu Thai has won every national election since 2001, including the last one in 2011. But its prospects for coming to power again have been clouded by constitutional and electoral system changes made since a 2014 coup that toppled a Pheu Thai-led civilian government

The new development came on the same day Prime Minister Prayut Chan-o-cha, who came to power in that coup, accepted the nomination to be the candidate for the Palang Pracharath Party.

Prayut said in a statement that he has accepted Palang Pracharath's nomination because he wants to work for the country.

"The decision was not easy because it is an important period for the country," he said. "I am ready to dedicate my life to protecting Thailand."

After the Thai Raksa Chart's nomination of the princess, the pro-junta People's Reform Party submitted a letter to the Election Commission asking that it be nullified on legal grounds, arguing that the much-revered monarchy cannot be used for political advantage in campaigning.

The princess posted a message on her Instagram account to "clarify that I had relinquished my royal title, and live as a commoner."

"Therefore, I would like to exercise my rights and freedom as commoner under the constitutional law," she said.

Palang Pracharat leader Uttama Savanayana declined to comment the princess' nomination, other than to say that everybody has such a right and that he is confident that the electorate will support Prayut's candidacy.

A political analyst who declined to be named told Kyodo News that this could be a turning point in Thai politics as it is unusual for a royal family member to run for the premiership.

The analyst said the upcoming election will essentially be a race between Prayut and Thaksin as it is widely known that the princess and Thaksin as well as his family are close friends.

"Those who do not want the royal family to run the country and those who hate Thaksin may turn their support to Prayut," the analyst said.

The princess, the eldest child of the late King Bhumibol, is one year older than King Maha Vajiralongkorn.

In 1972, while studying for her masters degree at the Massachusetts Institute of Technology, she relinquished her royal title to marry a foreigner. They had three children but later divorced, after which she returned to Thailand.

Her only son died in a 2004 tsunami that hit Thailand and other countries.

The Pheu Thai Party has nominated former health minister Sudarat Keyuraphan as its candidate for prime minister, while the Democrat Party has nominated party leader and former Prime Minister Abhisit Vejjajiva.

The five-day period for candidate registration for the lower house election ends Friday.