Thai Raksa Chart party leader Preechapol Pongpanich, holds up application of candidate for Prime Minister, Thailand's Princess Ubolratana Rajakanya Sirivadhana Barnavadi, at the election commission office in Bangkok, Thailand February 8, 2019. REUTERS/Athit Perawongmetha

BANGKOK (Reuters) - The leader of a pro-military party in Thailand said on Friday its rival’s nomination of the king’s elder sister as its prime ministerial candidate may breach election law.

Paiboon Nititawan, the leader of the People’s Reform Party, handed a letter to the Election Commission calling on it to decide whether to invalidate the nomination of Princess Ubolratana Rajakanya Sirivadhana Barnavadi.

He said the nomination could breach the law that prevents political parties from using the monarchy for campaigning.

Ubolratana relinquished her royal titles in 1972 after marrying an American fellow student, but she is still treated by officials and the Thai public as a member of the royal family.

The Election Commission is required to endorse or reject all candidates by next Friday.