Pavin Chachavalpongpun, a former Thai diplomat, gives regular lectures overseas criticising the junta and the Thai monarchy.
He draws admiration from many Thai dissidents for his no- nonsense commentary and has been a thorn in the junta's side since its 2014 takeover.
Thailand's military rulers revoked his passport in the wake of the coup.
They have also threatened to charge him with lese majeste, a crime that punishes criticism of the royal family with up to 15 years in prison.
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"It has nothing to do with me criticising the military, the main focus is on the monarchy," said Pavin, who works at Kyoto University but is currently a visiting scholar in the UK.
The alleged threats came after the professor gave two lectures at UK universities this week on the future of Thailand's monarchy, a sensitive subject as the kingdom's ailing 88-year-old monarch enters his twilight years.
Thailand's military leaders have maintained a tight lid on freedom of expression since their power grab, routinely summoning the country's few vocal critics to army barracks for "attitude adjustment" sessions.
Lese majeste prosecutions have jumped over the past two years under the ultra-royalist military junta.
Sixty-six people have been arrested for the crime since the coup, with the majority spending at least a year behind bars while awaiting trial, according to the International Federation for Human Rights (FIDH).
Those accused of lese majeste are often stained with a stigma in Thailand that can extend to their family members.