Thailand’s telecoms regulator rejected dtac’s request to develop 4G service on its unused 1.8GHz spectrum.

The National Broadcasting and Telecommunications Commission (NBTC) said the approval would violate the concession agreement between the operator and concession owner CAT Telecom, the Bangkok Post reported.

The country’s second largest operator, with a 30 per cent market share, had 50MHz of 1.8GHz spectrum under a 2G concession with CAT that expires in 2018. The company uses half of the total. Last year it returned 5MHz of the spectrum for the 4G auction, leaving it with 20MHz of unused airwaves belonging to CAT under the concession.

NBTC suggested that dtac show proof that both parties have agreed on the 4G development plan.

CAT acting president Sanpachai Huvanandana said he believed CAT could allow dtac to upgrade to 4G on the 1.8GHz spectrum because the unused bandwidth belonged to it under the deal, noting a reversal of the concession is unnecessary, the Post reported.

CAT is scheduled to discuss the issue with dtac next Monday.