BANGKOK (AFP) - A Thai folk singer-turned-activist was cleared of his fourth royal defamation charge on Friday (June 29), a rare acquittal in a country where the stringent law prevents even the mildest criticism of the monarchy and almost always results in conviction.

It is the second time that Tom Dundee, whose real name is Thanat Thanawatcharanont, has been acquitted this year after a separate charge was dropped in March.

But he remains in jail where he is serving a total of nearly 11 years for two other royal defamation convictions.

The lese majeste law carries up to 15 years in prison per offence and protects Thailand's ultra-rich and powerful monarchy from public scrutiny.

Critics say it is also often wielded against political dissenters with cases skyrocketing since an ultra-royalist military junta seized power in 2014.

Thanat was acquitted on Friday because prosecutors failed to prove he committed the crime, his lawyer told AFP.

"The court dismissed the case," Thamrong Lakdaen said.

Thanat was arrested in 2014 and hit with four counts of lese majeste over a series of critical YouTube videos and a speech at a political rally in 2011.

He cannot appeal his two convictions, for which he has already served three and a half years.

"Now all of his lese majeste cases are cleared," the lawyer said.

Lese majeste acquittals are very rare in Thailand where highly-sensitive trials are often held behind closed doors. The UN's rights watchdog says the conviction currently hovers around 90 per cent. The lese majestelaw is one of the toughest in the world.

In one high profile case earlier this year, five Thai teens and a 20-year-old were sentenced to between nearly 3.5 and 11.5 years in prison for setting fire to royal portraits.

Lengthy sentences have continued since Thailand's new king Maha Vajiralongkorn took power after his much-revered father died in 2016.

The new king, who spends much time outside the country, has not yet won the widespread popularity of his father.