The Criminal Court has ordered leading government foe Thanathorn Juangroongruangkit to remove allegations on social media that the state's coronavirus vaccine policy is opaque and unfairly favours a company owned by His Majesty the King, the government said on Sunday.
However, Mr Thanathorn’s Progressive Movement denied the videos were illegal and urged YouTube and Facebook to preserve freedom of expression.
The clips could still be viewed late on Sunday.
The Digital Ministry said the Criminal Court ruled that Mr Thanathorn's posts on social media and his movement's website could breach national security.
The videos accuse Prime Minister Prayut Chan-o-cha's government of lack of transparency in allowing Siam Bioscience, which is owned by His Majesty, to supply most doses despite a lack of vaccine experience.
Critics say former junta leader Gen Prayut rigged a vote in 2019 to retain power -- a charge he denies -- and have broken a national taboo by increasingly criticising the monarchy.
Siam Bioscience received a 600 million baht subsidy to develop capacity to manufacture AstraZeneca vaccines domestically and across Southeast Asia.
The court did not comment when contacted by Reuters.
Mr Thanathorn's movement said it had not received the ruling. "We continue to stress that the content is not false or a threat to national security," tweeted Pannika Wanich, a prominent Progressive Movement member.
"We hope YouTube and Facebook will stand by the rights and freedom of expression."
Mr Thanathorn was banned from politics for 10 years after a court dissolved his previous Future Forward Party last year over loans deemed illegal. He denied the charges.
Now the government has lodged a case against him over the video, accusing him of insulting royalty, a charge punishable by up to 15 years' prison.
The government says Siam Bioscience manufacturing is on track to deliver a first batch of 61 million doses to the public by June.
There was no immediate comment from Mr Thanathorn or Siam Bioscience on the court ruling. The palace traditionally does not comment on political disputes.
Thailand on Sunday reported 829 new coronavirus cases, bringing the total to 18,782 cases and 77 fatalities since the outbreak started last January.