A retired truck driver who was serving a 20-year sentence for insulting Thailand's king has died in a prison hospital.

Ampon Tangnoppakul, 61, was convicted in November after prosecutors claimed he had sent four text messages that defamed the Thai monarchy. He claimed he hadn't sent the messages – and that he didn't even know how to use the text function on his mobile phone. But he was found guilty and given the heaviest sentence ever handed down for violating Thailand's strict lese-majesty laws.

Ampon – known as "Uncle SMS" in the press – had a history of cancer and his lawyer said his condition worsened in prison.

Political abuse

Thai prosecutors have been getting increasingly aggressive about using the lese-majesty laws, and activists say they're used to intimidate and silence government opponents. A campaign by Thai academics and activists to change the law has been met with threats and violence.

Read more: Noam Chomsky and other foreign scholars support the call for change.