The group Asean Parliamentarians for Human Rights (APHR) said it is concerned Mr Porlajee’s disappearance is related to his long-standing efforts to assist fellow ethnic Karen villagers in the struggle for recognition of indigenous land rights in Kaeng Krachan National Park.
Mr Porlajee is involved in a lawsuit that accuses park officials of destroying the homes of more than 20 families in the area.
His efforts have put him in conflict with the park authorities there, which leads to suspicions he met with foul play.
“Police have a duty and responsibility to immediately investigate Billy’s disappearance and make every effort to establish his whereabouts and his movements since being detained by park authorities on April 17,” said Kraisak Choonhavan, vice-president of APHR, a former Thai senator and renowned human rights and environmental activist.
“Impunity, and efforts to cover up for fellow state authorities accused of wrongdoing continues to be a massive problem here.
"But the eyes of the region and the world are fixed squarely on the Thai authorities at this moment — there will be no getting away with the usual half-hearted investigations this time around.”
Mr Porlajee was detained and questioned after being found in the park in possession of wild honeycomb and six bottles of wild honey before his appearance.