The story of a former Thai king slaying a Burmese prince in a duel while riding elephants is at the centre of a police investigation against an elderly Buddhist intellectual who faces 15 years jail for questioning the historical accuracy of the event.

Sulak Sivaraksa, 85, made the comments at a university seminar in 2014 and was charged then, but the case has resurfaced, with police saying they have now finished their investigation.

This week police escorted Mr Sulak to meet the prosecutor, who will decide whether to go ahead with the case.

He is charged with lese majeste, which protects the king, queen, heir and regent from defamation, but has been more broadly applied to cover the king's dog, past kings and even Facebook "likes" of controversial material.

Mr Sulak insists he has done nothing wrong.

"My point is, if you want to learn history, you have to get all facts from the past as much as you can, and I just state the facts," he said.

The case comes just before the cremation of Thailand's widely-revered King Bhumibol Adulyadej later this month, but Mr Sulak said he was not sure why the charges had been revived.

"Well, I wish I knew," he said.

"The country is run by dictatorship so [there's] no regard to the rule of law and for the army, King Naresuan is a great hero, you see, and the elephant combat is real for them."

Mr Sulak is a renowned social critic and the founder of the International Network of Engaged Buddhists. He has been nominated twice for the Nobel Peace Prize.

His autobiography - Loyalty Demands Dissent - contains a foreword from the Dalai Lama.

Although he describes himself as a royalist, he has been charged several times with lese majeste.

Different versions of historic duel

The story of the elephant duel has been the subject of many movies and television series in Thailand and is held dear by the military, which currently runs the country.

"Duel on elephant back was supposed to be the height of warfare at that time," said Chris Baker, a Thai-speaking historian who has published history books and translations of poems.

A mural painting depicting the 16th century battle. ( Flickr: Andrea Kirkby )

"It's a bit like knights used to joust ... so it's very romantic to have an elephant duel."

According to the mainstream retelling, Thai King Naresuan challenged Burmese Crown Prince Mingyi Swa to a duel in 1593.

The two fought from the backs of elephants and King Naresuan killed Prince Mingyi Swa with his lance, according to the movie version.

Mr Baker recently published a book about this period of Thai history, so the source material is fresh in his mind.

"There's around 10 different accounts of the incident and it's important to state at the beginning that not one of these claims to be an eyewitness account," he said.

"So these are all just kind of stories about an event ... and these are all different, no two are the same and they differ about every single detail of the thing."

Mr Baker said even the location of the elephant battle is disputed.

"Then there's the question of whether ... it was a formal duel, whether Naresuan challenged the Burmese prince or in some accounts they just met in the middle of the battle and fought against one another.

"More importantly there's a question, big differences in how he is killed. [In] the mainstream version, Naresuan kills him with a sword. In other versions the Burmese prince is killed with a bullet."

Royal defamation for dead kings

Besides the question of historical accuracy, there is the application of the lese majeste law, which protects the king, queen, heir and regent from slander, but is now being used to shield dead kings too.

"We used to think it was pretty simply in that it identified who were the personages who could be insulted and therefore the law could be invoked," Mr Baker said.

"But it now seems to be much broader than that and I think it's very difficult to understand now."

On December 7, the prosecutor will announce whether to go ahead with the case against Mr Sulak.

If it does go ahead, the trial would be held in a military court, possibly in secret.

"So I'm at the mercy of the military tribunal... if they want to be fair, if they want to be sincere I'll get acquitted, but if they want to punish me, well, what can you do? There's no rule of law," Mr Sulak said.