The Commerce Ministry is investigating whether Kularb Kaew, the company used by Temasek Holdings of Singapore to purchase the majority stake in Shin Corp from Thaksin Shinawatra's family, has violated Thai ownership law.

If it is found to be illegal, foreign ownership in Advanced Info Service, the major mobile-phone operator, would exceed the limit of 49 per cent.

Suchai said he would address the issue after the ministry completes its investigation.

Pipop Udorn, a lecturer at Thammasat University, said authorities should focus the inquiry on circumstantial evidence to see if Kularb Kaew held the Shin Corp shares on behalf of Temasek.

That could include the amount of dividends paid to Thai shareholders. If the ratio is 3 per cent, as is the case now, the remaining 97 per cent of dividends must be paid to foreigners, making the firm an illegal entity, he added.