Bangkok, Thailand: A Thai woman was sentenced on Wednesday to nine and a half years in prison for royal defamation, the latest in a spate of prosecutions that have raised concerns about the junta's application of the draconian law.

The most recent high-profile cases include a probe into the US ambassador for comments he made about jail terms under the lese majeste law and charges against a man for allegedly insulting the king's dog.

Chayapa Shokpornbudsri, 49, was found guilty of defaming the monarchy on Facebook at a hearing without her lawyer present, a court official said Wednesday. 

She was handed a 19-year sentence which was cut in half after she pleaded guilty, the official said, adding it was ‘a secret ruling’.

‘From her Facebook posts, she was found guilty of defaming the monarchy, threatening state security and violations of the computer crimes act,’ the court official told AFP on condition of anonymity.

Thailand has one of the world's harshest lese majeste laws and prosecutions have soared since the army, which styles itself as the champion of the monarchy, grabbed power in a coup last year. 

Any member of the public can make an allegation of royal defamation, which the police are duty bound to investigate.

Critics say that situation often results in effective witch-hunts, led by ultra-nationalist volunteers who comb through social media and public events for possible breaches of the law.

Those convicted of insulting the revered but ailing 88-year-old King Bhumibol Adulyadej, or the queen, heir or regent can face up to 15 years in jail on each count.

Chayapa's lawyer Khumklao Songsomboon, from Thai Lawyers for Human Rights, told AFP Wednesday that she had not been informed of the court hearing in advance, as was customary.

Chayapa ‘was crying when she rang yesterday... I only knew about the verdict after it had happened,’ Khumklao said.

‘I don't know why (they didn't tell me). I don't know what the court's intention was,’ she said, adding it was a violation of her client's human rights.

Chayapa ‘was accused of posting some messages in June about a counter coup that could have created chaos in the country and which defamed the monarchy,’ she added.

Record-breaking sentences have been handed down in recent months to transgressors, most of whom are regime critics but also include senior officials swept up in nebulous corruption probes.

The US ambassador to Thailand faces investigation over a speech last month in which he expressed concern at the lengthy sentences under the law.

Two men were arrested this month for sharing an infographic on Facebook about a corruption scandal over a public park, and this week a man was charged with lese majeste for insulting the king's dog.