Amphon Tangnoppakul, who became known as Uncle SMS after he was convicted of defaming Thailand's royal family in a series of text messages, has died in a Bangkok jail.

The 62-year-old, who had four children, was suffering from mouth cancer. He was sentenced in November to 20 years in prison after he was found guilty under draconian defamation laws. In March he was refused bail while his lawyers sought to appeal against his conviction.

As news of Mr Amphon's death spread, several dozen demonstrators gathered outside Bangkok's Central Criminal Court holding signs that condemned the harsh lèse-majesté laws. His wife, Rosmalin Tangnoppakul, only learnt of his death while she was trying to visit him, according to the Associated Press.

"Amphon Tangnoppakul, you can come home now," she said, as she burned incense at the prison reception area and said prayers. "You're free now. Come home."

It was not immediately clear what the cause of death was, but reports said Mr Amphon had complained of stomach pains last week and had been transferred to the prison hospital. Earlier this year, his wife told The Independent that she had struggled to arrange check-ups with his cancer specialist since he had been sent to jail.

"The court said he cannot have bail because his case is connected to people's feelings," she said, after his application for bail was rejected.

Mr Amphon, a retired lorry driver, who was also a grandfather, was arrested in August 2010 and accused of using his phone to send four text messages to the private secretary of the then Prime Minister that were deemed offensive to the Queen. He denied sending them and said he did not know how the SMS function on his phone worked.

In court he pleaded not guilty and said his phone was being repaired at the time the messages were sent. The court was unimpressed because he could not identity the repair shop.

He wept during the trial proceedings, saying: "I love the King." His sentence was lengthened because of additional penalties handed down under a 2007 computer crimes act. The case has drawn fresh attention to the controversial lèse-majesté laws that critics say are being increasingly misused. A campaign to reform the laws has run into strong opposition from nationalist groups who say the laws are vital to protect 84-year-old King Bhumibol Adulyadej and other members of the monarchy. Campaigners say the laws have often been used to silence political opponents and dissidents.

The government of Yingluck Shinawatra, elected last summer, had said it would review the laws.

It seems however, that in face of the vociferous and often violent campaign to retain them, it will not for now expend political capital by trying to reform the current legislation.