A Thai court on Wednesday ruled out negligence in the treatment of a grandfather who died in jail of cancer while serving a controversial sentence for defaming the monarchy.

Ampon 'Akong' Tangnoppakul, considered a "prisoner of conscience" by rights groups, was convicted for sending text messages deemed insulting to the royal family in December 2011 and died in May the following year.

The Bangkok Criminal Court, which conducted the inquest because Ampon died in custody, said there was insufficient evidence of negligence.

It found that Ampon died due to the spread of liver cancer.

Fellow prisoners had told the court that Ampon had not received enough food or health care while he was in jail, a judge said, but concluded that his treatment was in line with other inmates.

Ampon's conviction triggered rare public protests against Thailand's lese majeste law.

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

Ampon, who became known in Thailand as "Uncle SMS", was sentenced to 20 years after pleading not guilty during his trial, one of a series under the royal defamation legislation which critics say is used to stifle free speech.

His family are now pushing for a higher standard of care to be adopted for all prisoners, said their lawyer Poonsuk Poonsukcharoen, adding that the prison hospital had inadequate staff and equipment.

"He was bedridden for three days before he died. If the health care was up to standard, he should have been diagnosed earlier," she told reporters, adding the family would seek compensation through the courts.

The royal family is a highly sensitive subject in the politically turbulent kingdom.