A group of ultra-royalists have accused a prominent Thai historian of defaming the monarchy by raising doubts about a historic elephant battle between an ancient Thai King and a Burmese general 400 years ago. 

The royalists filed a lese majeste (insult of monarchy) complaint against the author, Sulak Sivaraksa, at Chanasongkram Police Station in Bangkok today, according a report on Chao Praya news site.

In Thailand, criticising the monarchy is punishable by up to 15 years in prison under Section 112 of the Thai Criminal Code.

Lt.Gen. Padung Niwetsuwan, a member of the group, said the 82-year-old writer insulted the Thai monarchy by questioning the accomplishments of King Naresuan, who ruled ancient Thailand - known then as Ayutthaya Kingdom - between 1590 and 1605.

According to Lt.Gen. Padung, Sulak suggested at a recent academic forum that the elephant duel between King Naresuan and the leader of the invading Burmese army, Minchit Sra, may not have happened, contrary to the account in official Thai history.