BANGKOK (AFP) - The governor of Thailand's Chiang Mai province has sued a local magazine for posting a "blasphemous" painting on Facebook of ancient kings wearing pollution masks as part of a campaign to protest the city's hazardous smog.

All matters touching on the monarchy are highly sensitive in Thailand, a country where kings have been worshipped as near deities and are protected by one of the world's harshest royal defamation laws.

The broadly-interpreted crime of lese majeste - which can carry decades-long sentences - has cemented a culture of self-censorship across the kingdom's academic, media and arts circles when it comes to royal affairs.

The risks of testing those boundaries were on display on Friday (March 30) when an English-language magazine faced legal action for posting an image on social media of three statues of ancient kings - a Chiang Mai landmark - in pollution masks.

The painting, which was the work of a local high school student, was posted on Citylife Chiang Mai's Facebook page to promote a rally urging authorities to tackle a toxic haze that plagues the northern city annually.

The province's governor called the artwork a "blasphemous act" and dispatched an official to file charges against the outlet under Thailand's Computer Crime Act.

The cyber-crime law, which carries up to five years in prison for uploading false content to the web, is routinely used against perceived critics of the monarchy on social media, though it is not as harsh as the lese majeste law that carries up to 15 years per offence.

"I assigned my official to file a complaint with police yesterday that the picture may have violated the Computer Crime Act as it's inappropriate," Chiang Mai governor Pawin Chamniprasart told AFP on Saturday.

"The statues of three kings are very sacred and respected by Chiang Mai residents, they were our ancestors," he added.

In an official letter to police, the governor said the painting "may affect Chiang Mai's image and its tourism, causing the city economic instability".

Chiang Mai is one of Thailand's largest cities and a major hub for travellers exploring the country's lush and mountainous north.

But it also struggles with dangerous levels of air pollution during crop-burning season.

Police confirmed they were investigating the case, while the magazine announced that its 'Right to Breathe' protest had been cancelled.

In a Facebook post the teenage artist behind the painting said it was "a shame that people are hurt by a picture and not the polluted air that they are breathing in".

Freedom of expression has been severely restricted in Thailand ever since a 2014 coup installed an ultra-royalist junta that has stamped out dissent and hounded monarchy critics.

Prosecutions under lese majeste and Computer Crime Act have shot up under their rule, often netting social media users.