Half of DTAC's spectrum of 50 MHz on the 1800-MHz frequency under its concession with CAT Telecom is unused. The concession is due to expire in 2018.
Both DTAC and CAT want to use it to launch their 4G services.
Gp Capt Anudith Nakornthap, the ICT minister, earlier supported CAT's move but DTAC's chief refused to return the unused spectrum to CAT as ordered by the minister.
DTAC argued that now that the National Broadcasting and Telecommunications Commission (NBTC) is the proper regulator, any unused spectrum must be handed over to the NBTC to be auctioned in the best interests of the public.
The country's second-largest mobile operator proposed two options to the NBTC and the ICT Ministry.
One is to let it provide 4G services on the spectrum under the existing 2G concession with CAT. The other is for it to return the spectrum to the NBTC for auction.
"I personally think CAT should allow DTAC to provide 4G on the unused spectrum now, instead of leaving it unused through the concession life. The issue should be settled within this year," Gp Capt Anudith said.
He said he supported DTAC's proposal to provide 4G services on its unused portion of the bandwidth if the move could benefit the telecoms industry and generate more revenue for the state telecom enterprise.
But Gp Capt Anudith said CAT and the NBTC should clarify the issue, particularly those areas that risk a breach of contract.
DTAC chief executive Jon Eddy Abdullah insisted last week that the company could run 4G services under the existing concession, saying mobile operators were earlier allowed to upgrade their 2G networks to 3G using high-speed packet access technology.
The concession terms allow the company to install equipment and operate mobile services that are equal to 2G or better, according to Mr Abdullah.
Mr Abdullah said operating 4G on the unused spectrum is practical, given its large band slot to accommodate mobile data traffic. In comparison, he said the current 15 MHz of the 2100-MHz frequency that now provides 3G service is not enough for 4G.
However, he acknowledged that the company's 4G proposal might be an uphill task as even if it gets approval from CAT to move ahead with its 4G plan on the existing concession, the state enterprise could not share the revenue.
CAT cannot book all related concession revenue into its financial statement from Dec 20 this year under the 2010 Frequency Allocation Act.
CAT chief executive Kittisak Sriprasert said no resolution on this issue has been made by the state enterprise's management as it needs to hold more detailed discussions with all related parties.
But he stressed that what CAT wants to know is whether the state enterprise will benefit from DTAC's proposal.
CAT will propose potential solutions to the NBTC and the regulator will make a decision.
Mr Kittisak said he believed the 4G proposal might lead to an amendment of the 2G concession contract.