Dtac, the third largest mobile operator in Thailand, is considering legal action if the regulator doesn’t allow it to continue using the 850MHz spectrum band after its concession with CAT Telecom expires on 15 September, Bangkok Post reported.

A source told the newspaper the board of the National Broadcasting and Telecommunications Commission (NBTC) is not expected to approve dtac’s request to temporarily continue using the 2G spectrum to give it time to migrate all its customers to other networks, as stipulated by the original concession terms.

A dtac executive said 380,000 customers remain on the 850MHz network but noted all of its 21 million customers could face disruption as the band is used for roaming to other networks.

The source said NBTC likely faces legal action even it it allows dtac more time as rivals AIS and True Move could file lawsuits claiming favouritism, Bangkok Post reported.

To protect it from complaints from other operators, NBTC passed a resolution on 2 July requiring dtac to participate in the 900MHz auction if it wanted an extension to the concession period. Dtac opted not to participate in the auction, and the regulator was forced to cancel the sale as the other operators showed no interest.

Dtac and market leader AIS each bid THB12.5 billion ($375 million) for 10MHz blocks in last month’s 1.8MHz spectrum auction. The long-delayed 4G sale ended with only two of nine blocks selling.