A Thai billionaire opposition figure was accused Wednesday of defaming the monarchy, a day after a court sentenced a former civil servant to more than 40 years in jail for lese majeste crimes.

Thanathorn Juangroongruangkit, founder of the now-dissolved Future Forward Party, was accused of contravening Thailand's strict royal defamation laws by posting a video Monday criticising the government's vaccine strategy.

In the video he raised questions about whether Thailand's vaccination campaign is too reliant on Siam BioScience, which is owned by the Crown Property Bureau -- a body that manages the royal family's multi-billion-dollar fortune.

His video drew the ire of Thailand's Ministry of Digital Economy and Society, which on Wednesday filed a royal defamation complaint against him.

"What Mr. Thanathorn said is not the truth; the facts have been distorted," ministry representative Tossapol Pengsom told reporters.

He said there were 11 instances in the video where the opposition figure allegedly insulted the monarchy, and the ministry would be file a complaint under the Computer Crimes Act.

The lese majeste law, referred to as "112" by its penal code section, is meant to shield the royal family from defamation, and carries sentences of between three to 15 years in jail per charge.

It has, however, been broadly interpreted to cover any perceived criticism of the monarchy, with some saying the law is also used to target government dissent.

On Tuesday, a court sentenced a former civil servant arrested in 2015 to more than 43 years in prison for disseminating audio clips deemed defamatory to the monarchy -- the harshest punishment ever received.

The law's use slowed in 2018, which Premier Prayut Chan-O-Cha said was due to the "mercy" of the king.

But it returned in full force last year, in response to mass demonstrations staged by a pro-democracy movement calling for Prayut's removal from office and -- more controversially -- for reforms to the unassailable monarchy.

Billionaire Thanathorn Juangroongruangkit has been accused of defaming Thailand's monarchy, after a ministry filed a criminal charge against him
Billionaire Thanathorn Juangroongruangkit has been accused of defaming Thailand's monarchy, after a ministry filed a criminal charge against him AFP / Lillian SUWANRUMPHA

So far, at least 43 protesters have been charged for their role in the protests.

The kingdom appears to now be going through a backlog of stalled lese majeste cases.

Thanathorn -- who threw his support behind the movement when it started in mid-July -- is the highest-profile target since the spike in the law's use.

"It means that the draconian law is in full swing and can be used indiscriminately," said political analyst Thitinan Pongsudirak, of Chulalongkorn University.

Pannika Wanich, Thanathorn's de facto spokesperson, told AFP it was "obvious" the law is was being deployed "aggressively against the government's political enemies".

Government spokesman Anucha Burapachaisri did not respond immediately for comment.

Before it was disbanded, Thanathorn's Future Forward Party was Thailand's third-largest, drawing millions of supporters who were attracted to his anti-establishment platform during the 2019 election.

But they were felled by swift legal action, which saw its top executives, including Thanathorn, banned from politics and the party dissolved.