Thailand's King Maha Vajiralongkorn attends the annual Royal Ploughing Ceremony in central Bangkok, Thailand, May 14, 2018. REUTERS/Athit Perawongmetha

BANGKOK (Reuters) - Thailand’s King Maha Vajiralongkorn said on Friday his elder sister’s announcement she is running for prime minister in March elections is “inappropriate” and unconstitutional, likely sinking her candidacy for a populist opposition party.

Princess Ubolratana Rajakanya Sirivadhana Barnavadi, 67, shocked the nation when she announced on Friday she would be the sole prime ministerial candidate for the party, which is loyal to ousted ex-premier Thaksin Shinawatra.

But the opposition from her younger brother, a constitutional monarch, is likely to lead to her disqualification by the Election Commission.

“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.