"There are two electricity producers in the industrial estate. A merger would leave only one operator, which would reduce competition with significant implications. As such, the rejection of the merger ... is in accordance with the law," Narupat Amornkosit, secretary-general of the Energy Regulatory Commission, said in a statement.

ERC had in October blocked the $4 billion merger deal proposal and GPSC had appealed against it last month.

(Reporting by Chayut Setboonsarng; Editing by Rashmi Aich)