The military-run government of Thailand has announced a ban on all online interaction with three of its most prominent overseas critics.

A letter from the digital economy and society ministry warned citizens that engaging on the internet with the Thai academics Somsak Jeamteerasakul and Pavin Chachavalpongpun as well as the journalist Andrew MacGregor Marshall could violate the law.

All three live outside Thailand but have large online followings in the country for their commentary about the failings of both the junta and the monarchy.

The ministry statement said citizens should not follow, contact or share content from the trio on the internet or social media. The letter added that people who disseminate their information, directly or indirectly, could be violating the country’s Computer Crime Act.

Historian Somsak has written extensively about the Thai monarchy and Pavin is an outspoken former diplomat. Marshall is the author of A Kingdom in Crisis, published in 2014, which is banned in the country.

Open discussion of the king is illegal under Thailand’s strict lese-majesty laws and some critics have been handed decades-long jail sentences.

Thai authorities had previously warned that even Facebook shares could be considered a violation of the lese-majesty law. A student activist, Jatupat Boonpattararaksa, was charged in December for sharing a BBC profile of the new king on Facebook. Jatupat remains in jail having been denied bail.

The Thai government also makes efforts to take down any online material it considers defamatory against the monarchy. Shortly after the death of the king in October last year, a high-level Thai delegation met Google to push the company to remove any anti-monarchy content. The Daily Mail is blocked in Thailand for what was perceived as unfavourable reporting on the royal family.

There has been a surge in lese-majesty prosecutions under the junta, which Pavin argued in a 2015 opinion piece for the Guardian was an attempt to “seek legitimacy in order to prolong its rule”.

Josef Benedict, Amnesty International’s deputy director for south-east Asia and the Pacific, said Thai authorities had “plunged to fresh depths” in restricting people’s freedoms of expression with the new ban.

“After imprisoning people for what they say both online and offline, and hounding critics into exile, they want to cut people off from each other altogether,” he said. “The move doesn’t reveal strength, but a weakness and fear of criticism. In its determination to silence all dissent, the Thai authorities are resorting to extreme measures that brazenly flout international human rights law.”

A senior official at the digital economy ministry denied the new order raised the bar for repression in the country. “This is to benefit the people so they can search for the right information … and use their judgment so that it [the order] will not affect them,” Somsak Khaosuwan said after the announcement.

Sensitivity over any activity deemed as anti-monarchy has grown acute since King Maha Vajiralongkorn ascended the thrown following the death of his father, Bhumibol Adulyadej.

All media based in Thailand self-censors to stay within the law. That has made it impossible to objectively report on the reign of the new king, who has added clauses to the country’s new constitution that bolster his powers. King Vajiralongkorn spends much of his time in Germany.

The Associated Press and Agence France-Presse contributed to this report