Thailand's Election Commission has disqualified the sister of the country's king from becoming a candidate for prime minister in the March general election.

In a statement read on all Thai television stations within hours of Ubolratana's announcement as a candidate, King Vajiralongkorn said it was "inappropriate" and unconstitutional for members of the royal family to enter politics.

Thailand will hold elections on March 24, the first since a 2014 military coup.

Princess Ubolratan did not comment on her prime ministerial candidacy, but thanked her supporters and said she wants Thailand to be "moving forward".

Upon Princess Ubolratana's permanent return to Thailand after her divorce in 1998, she was "bestowed the title "Tunkramom Ying" (Daughter to the Queen Regent) title", and has since been "treated by officials as a member of the royal family". The commission is likely to follow the wishes of the monarch, who holds a semi-divine place in Thai society.

"The royal announcement made it clear that the party violated electoral law", said Mr Srisuwan Janya, secretary-general of the Association for the Protection of the Constitution.

"The EC must therefore take steps to dissolve the party", Paiboon said, citing Section 92 of the 2018 Political Party Act's stipulation on dissolution of the party when it has obtained credible evidence that it has committed an act deemed hostile to constitutional monarchy rule.

Shortly after the king's statement she posted again without addressing the issue directly, simply thanking Thais for their support and saying that she wanted Thailand to "move forward and become admired and accepted by the worldwide community".

Meanwhile, a chastened Thai Raksa Chart, a key pillar in Thaksin's election strategy, agreed to comply with the royal command.

Her shock nomination broke with a long-standing tradition of members of the royal family staying out of politics.

"All members of the royal family must abide by the king's principle of staying above politics, maintaining political impartiality and they can not take up political office", part of the statement released to the media said.

Parties loyal to former telecommunications tycoon Thaksin have defeated pro-establishment parties to win every election since 2006, but every time have been removed by court rulings or coups.

"Things are now more unpredictable", Titipol told Reuters.

"I would like to say once again that I want to see Thailand moving forward, being admirable and acceptable by worldwide countries, want to see all Thais have rights, a chance, good living, happiness to all", she said, ending her post with an "#ILoveYou" hashtag.

Thaksin has lived in self-imposed exile in England since he was deposed.