A Thai court on Wednesday sentenced a man to 20 years in prison for sending text messages deemed insulting to the monarchy, his lawyer said, under the kingdom's strict lese-majeste laws.

Ampon Tangnoppakul, 61, was found guilty of sending messages to the private secretary of then-prime minister Abhisit Vejjajiva on at least three dates in May 2010, according to the criminal court in Bangkok.

"The court found him guilty and sentenced him to 20 years in jail," his lawyer Anon Nampa told AFP, adding that he had 30 days to lodge an appeal.

Ampon was arrested in August last year and pleaded not guilty to the charges during his trial.

After his arrest, Thailand's Central Bureau of Investigation said the messages were "inappropriate and considered insulting to the monarchy and have upset the recipients," without revealing their content.

The royal family is a very sensitive subject in Thailand. King Bhumibol Adulyadej, 83, is the world's longest-reigning monarch and revered as a demi-god by many Thais.

Under Thai laws, anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.