The Supreme Court’s Criminal Division for Political Office Holders had previously sentenced him to 42 years, but Boonsong appealed the verdict.
However, he lost the appeal as the panel of nine judges decided to uphold the previous verdict and added another six years to his sentence.
The court also convicted 14 other co-defendants involved in one of the largest corruption cases in Thailand’s history.
The verdict is related to the government-to-government rice scandal and the case was initiated after the military overthrew Yingluck’s government in the 2014 coup.
The other key defendants who were given between four and 48 years in prison by the Supreme Court include former deputy commerce minister Poom Saraphol, former Foreign Trade Department chief Manat Soithong and his deputy Tikamporn Natvoratat, Boonsong’s former secretary Veeravuth Watjanabukkha, and well-known rice trader Apichart “Sia Piang” Chabsakulporn of Siam Indiga Company.
The Supreme Court also handed down prison terms and fines to several other rice millers.