The Thai Raksa Chart Party threw election politics into chaos on Friday when it nominated Princess Ubolratana as its candidate to be prime minister.

The party officially registered the elder sister of His Majesty King Maha Vajiralongkorn at the Office of the Election Commission.

The move was the focus of election rumours on Thursday and overnight, but no media had been bold enough to mention her name or the royal connection until the actual event at 9am Friday.

Executives of the party arrived at the EC office at the Government Complex after announcing they would have only one candidate for prime minister - what they called "an important person outside the party".

As they checked in at the EC office, they showed reporters a document confirming Princess Ubolratana as their candidate as prime minister.

The move is a serious blow to plans by Prime Minister Prayut Chan-o-cha and the Palang Pracharath Party (PPRP) to extend power via the March 24 elections.

PPRP officially registered Prime Minister Prayut Chan-o-cha as its prime ministerial candidate, but the news of the nomination of Princess Ubolratana completely overwhelmed his nomination.

Thai Raksa Chart is a spinoff from the Pheu Thai Party, and both are friends of the fugitive former prime ministers Thaksin and Yingluck Shinawatra.

Princess Ubolratana has been seen with the Shinawatra brother and sister in public, notably at the most recent World Cup tournament in Russia.