A court in Thailand has sentenced a man to 20 years in prison for sending text messages that authorities say were “insulting” to the country's royal family.

Bangkok's Criminal Court on Wednesday found 61-year-old Ampon Tangnoppakul guilty of sending four “inappropriate” messages in May of last year to the secretary of then Prime Minister Abhisit Vejjajiva.

Authorities did not reveal the content of the messages, but said they were “considered insulting to the monarchy and have upset the recipients.”

Ampon, who is known in Thailand as “Uncle SMS,” has maintained his innocence, saying he did not possess the SIM card used to send the messages.

He was arrested in August of last year and charged with four violations of the country's Computer Crime Act and its Criminal Procedure code, which criminalizes insults to the monarchy.

Some observers say Thailand has seen a recent increase in convictions for those said to have defamed the royal family.

Last month, a Thai-born U.S. citizen pleaded guilty in a Bangkok court to charges of insulting the monarchy.

Joe Gordon, a 55-year-old resident of Colorado, was arrested in May while vacationing in Thailand.

Gordon is charged with posting material deemed offensive to the royal family on his blog, as well as a link to a translation of a banned book, while he was living in the U.S.

Following the incident, Frank LaRue, the United Nations' special rapporteur on freedom of expression, urged Thailand to amend the controversial laws, saying they encourage self-censorship and stifle important debates on matters of public interest.

He added that the laws are overly broad and do not conform with the country's international human rights obligations.