Jailed opposition lawmaker Um Sam An is due to file an appeal this morning against his “unlawful” midnight arrest, according to his lawyer.

Defence lawyer Choung Choungy said he would apply to have the arrest deemed “illegal”, in light of his client’s parliamentary immunity, which had not been stripped by the National Assembly.

“The arrest of my client – since the beginning – has been illegal,” he said. Sam An was arrested a little after midnight on April 11 in Siem Reap, hours after flying back to Cambodia from the US.

He was questioned by anti-terrorism police and charged with “incitement” in the wake of his criticism of the government’s Vietnam border policy, claiming Cambodia had ceded territory to its neighbour.

While lawmakers can be arrested without first being stripped of their immunity in the case of “red-handed” crimes, Choungy claimed this was not the case for Sam An.

However, Ministry of Interior and Ministry of Justice spokesmen have both previously said that Sam An’s alleged posting of parts of a “fake” border treaty online, amounted to just such a “red-handed” case.