Nok’s shares fell more than 12 percent and Asia Aviation’s prices slid nearly 3 percent in the morning trading session.
Asia Aviation said in February that it was in talks to buy Nok shares, although Nok had said at the time that it was “not aware of any details in this respect”.
Nok is 53 percent owned by the Jurangkool family, which also controls Thai Steel Cable PCL and unlisted auto parts maker ThaiSummit.
Intense competition among budget airlines has led to quarterly losses since 2015 for Nok.
Asia Aviation owns 55 percent of Thai AirAsia, with the remainder held by Malaysia’s AirAsia Group Bhd.