Jailed ex-opposition figure Meach Sovannara has written a letter congratulating Prime Minister Hun Sen on his recent wedding anniversary in an apparent bid to secure a reduction in his 20-year prison term.

Sovannara has spent the past three years in jail, and was sentenced to two decades behind bars for leading a so-called “insurrection” at Freedom Park after a demonstration there turned violent in 2014.

“Highly respected Samdech! Prisoners of conscience are like a poor bird in a cage waiting for someone with divine power,” he wrote, adding that he and 14 other prisoners have nothing to offer the premier but “a drop of blood in this dark prison”.

“I almost forget the birthday of my wife and my youngest beloved daughter, but I, Meach Sovannara, still remember the special day for you Samdechs, the 5th of January which transformed from the days of struggling together to be a day of happiness together.”

The dual Cambodian-US citizen’s letter comes more than two months after the release of former opposition Senator Hong Sok Hour, who wrote a conciliatory letter to the premier last September apologising for posting an inaccurate version of a border treaty with Vietnam, which landed him in prison for “forgery” and “incitement”. Sovannara’s missive, however, stops short of making an apology.

Sovannara’s assistant, Chhim Kim Try, verified the authenticity of the prison letter, adding it was intended as a “soft” request for a pardon. “In reality, his attempt was to get a pardon, but he cannot make a request frankly,” Kim Try said. “The meaning is that he is playing soft to look for a solution.”

Council of Ministers spokesman Phay Siphan said any pardon was “up to the prime minister”, even though such pardons are formally signed by the King, adding that there are no political prisoners in Cambodia – not even jailed Cambodia National Rescue Party President Kem Sokha, who was arrested on allegations of “treason” last September. The party, which Sovannara also belonged to, was dissolved in November for allegedly fomenting “revolution”.

“They are put in jail according to their own motives if they do something against the law,” Siphan said. “Some groups see political motives, but the judge sees something different.”

Siphan suggested Sovannara would have to serve at least a third of his 20 years in prison before he would be considered for a pardon, but legal expert Sok Sam Oeun said pardons had been delivered with no time served – for example, the pardon of Funcinpec President Prince Norodom Ranariddh in 2008.

“I don’t think that just this letter can help, unless there is somebody to lobby for him . . . some close person with the family of the prime minister, or maybe embassy people,” Sam Oeun said, adding that it could “depend on the political situation and foreign relations pressure”.

Political analyst Ou Virak said he “wouldn’t be surprised” if Sovannara were released, despite the current anti-American sentiment and opposition crackdown.

“Strategically, the opposition movement has been neutralised. It’s time to just make sure there [are] no major reactions or sanctions from the international community,” Virak said.

“Releasing political prisoners would make sense. There needs to be . . . calm now. Prisoners’ release before the election is not unprecedented,” he added, referring to this year’s scheduled national ballot. “In fact, it’s been the case in previous elections.”