The source made the remark after Thosaporn, who was also an independent director, sent in his resignation letter with effect from October 15, seven months after his appointment on April 8 this year.
The source said Thosaporn faced difficulties in his work as he stood against government officials who hold political power.
"Thosaporn had a severe conflict with the government working teams," the source said, adding that he was pressured to resign from many positions that he holds.
The source added that Thosaporn plans to resign from all positions in the government. He is currently also the chief economic adviser to Prime Minister Prayut Chan-o-cha.
Thosaporn is considered an adviser trusted by Prayut. He had been appointed as National Economic and Social Development Council's secretary-general instead of Porametee Vimolsiri, who became permanent secretary at the Social Development and Human Security Ministry.