A Thai court on Tuesday jailed a mentally ill 65-year-old woman for one year for allegedly defaming the monarchy by insulting a portrait of the nation's revered king.

Thitinan Kaewjantranont, a so-called Red Shirt supporter of the toppled former government, was accused of "inappropriate action against a portrait of the king" at the Constitutional Court in northern Bangkok on July 13, 2012, with no further details given.

Initially Thitinan was given a suspended sentence for the 2012 offence as she was deemed to suffer from mental health problems, but the Court of Appeals rolled back the one-year suspended sentence Tuesday after prosecutors appealed.

"The defendant's behaviour was so evil... her actions should not be imitated by others, so the Court of Appeals lifted her one year jail term suspension," a judge said, reading the ruling.

Recognising she suffers from mental health issues, the court said she is ordered to see doctors every six months, without giving details of her condition.

Thailand's monarchy is protected by one of the world's harshest lese majeste laws, and convictions have surged since a military coup last May.

Under Section 112 of Thailand's criminal code, anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.

In late April the International Commission of Jurists -- an international human rights group -- said at least 49 people have fallen foul of the royal defamation rules since the coup, including those investigated, detained, convicted or awaiting verdicts.

Critics of the law say it is used as a weapon against political enemies of the royalist elite.

Reporting lese majeste cases is fraught with difficulty and media must heavily self-censor. Even repeating details of the charges could mean breaking the law.