Thai authorities are investigating the BBC over a Thai-language profile of the new king, officials said Wednesday, after the article outraged ultra-royalists in a country where criticism of the monarchy is outlawed.

A harsh royal defamation law has been used to jail scores of critics and spawned a culture of self-censorship across the kingdom's media, academia and the arts.

The BBC is now under scrutiny for the profile published out of its London offices of King Maha Vajiralongkorn, who ascended to the throne last week.

The royal succession was Thailand's first in seven decades and is considered a delicate moment for the institution. Vajiralongkorn does not command the respect enjoyed by his late revered father.

The BBC profile included information about Vajiralongkorn's personal life that is well-known inside the kingdom but rarely printed by Thai media, such as details about his three marriages that ended in divorce.

The article, which has since been blocked online in Thailand, quickly went viral on social media but attracted a torrent of criticism from royalists.

Any member of the public can allege royal defamation and authorities are duty bound to investigate.

On Tuesday a team of police officers visited the BBC's Bangkok office but found it closed.

"This case is in the process of investigation but I cannot disclose the details," Pornchai Chalordej, the commander of the Bangkok police station responsible for the case, told AFP.

Top leaders from Thailand's royalist junta, which has ramped up use of the lese majeste law since the 2014 coup, defended the probe on Wednesday.

"As they have an office in Thailand and Thai reporters work there they must be prosecuted when they violate Thai law," junta chief Prayut Chan-O-Cha said of the BBC.

The junta's number two, defence minister Prawit Wongsuwon, stressed the police's duty to investigate any wrongdoing.

"If anything is illegal we must prosecute accordingly without exception," he told reporters.

The BBC defended the profile in a statement and said its Thai-language service was committed to bringing "impartial, independent and accurate news to a country where the media faces restrictions".

"We are confident that this article adheres to the BBC’s editorial principles," it added.

On Saturday a prominent Thai dissident was arrested for sharing the BBC profile on Facebook.

Jatupat "Pai" Boonpattararaksa, a leader of a small group of anti-junta students, was released on bail but faces up to 15 years in prison if convicted of lese majeste.

The BBC's Thai service is one of few platforms that publishes comparatively unfiltered reporting in the Thai language.

Its articles are edited and published out of London, though the service employs a small number of local staff in Bangkok.

The BBC's English-language bureau in Bangkok is an editorially separate entity.