A Thai fortune teller arrested as part of a high-profile royal insult investigation has died, the second suspect in as many weeks to die in police custody.

Suriyan Sucharitpolwong, a well-known soothsayer and former aide to Crown Prince Maha Vajiralongkorn, died of natural causes, the Department of Corrections said.

The 53-year-old was found unconscious in his cell and taken to a hospital where attempts to revive him proved unsuccessful.

Known by the nickname Mor Yong, he had been charged with lese majeste, which in Thailand is one of the strictest such laws in the world, along with two other people involved in an alleged conspiracy to exploit the name of Crown Prince Vajiralongkorn.

Thailand's royal insult laws carry up to 15 years in prison for each count for anyone convicted of insulting the king, queen, heir or regent.