BANGKOK (REUTERS/AFP) - Thai police are investigating stickers lampooning Thailand's royal family that appeared briefly on the popular Japan-based Line instant messaging service, a police spokesman said on Thursday (April 7).

"We are investigating where the stickers came from and who did this," said Colonel Somporn Daengdee, deputy chief of the police's Technology Crime Suppression Division.

He declined to say more due to the sensitivity of the issue.

Line, which is one of Thailand's most popular social media platforms, apologised for the set of cartoon stickers and said it was no longer available for purchase.

The stickers had spread quickly online on Wednesday.

"Line Corporation is aware of the culturally sensitive sticker set that may have caused discomfort among our users in Thailand," Line said in a statement posted online. "The sticker set in question has been pulled from the Line Sticker Shop."

Cached webpages of Line's "sticker store" seen by AFP on Thursday show the set of cartoon images was available at 5pm GMT on Tuesday, but have now been deleted.

Most of the stickers referenced scandals and rumours that have trailed the monarchy despite efforts to control its image inside the kingdom.

Other more benign stickers showed royalty partaking in hobbies often publicised by the palace's media arm, such as King Bhumibol Adulyadej playing a saxophone.

Thailand's royal family is protected by some of the world's strictest lese-majeste rules, carrying a 15-year jail sentence pe r offence.

Under the military government which seized power in a May 2014 coup, prosecution of those deemed to have insulted the monarchy have risen rapidly and sentences become increasingly harsh.

It specifically outlaws criticism of the king, queen and his anointed successor, but is broadly interpreted to silence even tangential references members of the family and the institution.

King Bhumibol Adulyadej, 88, is a revered figure in Thailand. The King has been staying at a Bangkok hospital since May 2015 and worries over his health and the succession has formed the backdrop to more than a decade of political crisis in Thailand.

News about the King's health is tightly controlled by the palace which issues statements detailing his ailments.

Public debate about the monarchy is curtailed by the lese-majeste laws.

Those found guilty of breaking the royal insult laws face up to 15 years in prison. Rights groups say the law is often abused, contributing significantly to the deterioration of Thailand's rights record.

Line is the most popular messaging platform in Thailand. It allows users to compose their own stickers and sell them online. Many users include cartoon-like stickers in their messages.

Users can create and upload stickers that are checked by the head office in Japan and not in Thailand, said an employee at Line in Thailand who declined to be named because of the sensitivity of the matter.

The royal family stickers were created by a user, not the company.

On its website, Line says it "reviews (sticker) submissions against our sales criteria" before they go live.