A military court in Thailand has sentenced a woman to nine-and-a-half years in prison for defaming the country's monarchy on Facebook.

The defendant, identified as Chayapha, was arrested in June and charged with sedition for a posting allegedly opposing the military government.

A strict lese majeste law makes it a crime to criticise Thailand's royals.

Prosecutions have soared since the army, which styles itself a champion of the monarchy, seized power last year.

Critics have said the broadly-worded lese majeste laws are being used to silence dissent and opponents.

The woman was found guilty on Tuesday at a hearing at which she did not have a lawyer present, a court official told AFP news agency.

"From her Facebook posts, she was found guilty of defaming the monarchy, threatening state security and violations of the computer crimes act," the official was quoted as saying.

The single mother's 19-year sentence was cut in half because she pleaded guilty.

Her lawyer Thanathorn Thananon said he had only heard of the sentence after it was handed down.

"The defendant was originally called in and charged with sedition for posting on Facebook that a counter-coup against the junta was imminent," he said, the Bangkok Post reports.

"Further investigation by the authorities found content that brought about lese majeste charges.

The military courts failed to inform us that a hearing was taking place. They only told the defendant to appear at short notice."

Just press 'Like'

Since its coup last year, the military government has arrested a string of suspects, accusing them of claiming or using connections to the monarchy for personal benefit.

"Since Thailand's coup, it hasn't been hard to get thrown in jail for criticising the junta," Brad Adams, Asia director at Human Rights Watch, told Reuters, referring to another lese majeste case.

"Now, all you have to do is press 'Like' on your Facebook page."