A Thai grandfather sentenced to 20 years in prison for sending text messages that were considered insulting to the country's royal family has died while in jail.

The lawyer of Ampon Tangnoppakul said Tuesday his 62-year-old client died at a correctional hospital after complaining of stomach pains. His cause of death is not known.

In November, Ampon, who became known as “Uncle SMS,” received one of the toughest ever prison sentences handed out under the country's strict lese majeste laws.

The court found him guilty of sending four messages to a government official that were deemed offensive to the queen. Ampon pleaded not guilty, saying he did not send the messages.

Observers have reported a recent increase in the number of people prosecuted for defaming the monarchy. Human rights groups have urged Thailand to amend its toughest in the world lese majeste laws, saying they are overly broad and do not conform with the country's international human rights obligations.