Speculation over the deal, which saw Prime Minister Thaksin Shinawatra's family and relatives sell their 49.6 percent stake to a group led by Singapore's Temasek Holdings, had pushed Shin shares up 25 percent since December.

"Our committee is investigating the trading of Shin shares and people involved in any insider trading," exchange president Kittiratt Na Ranong told reporters late Monday.

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Shin shares closed unchanged at 48.25 baht (HK$9.56) Tuesday .

The deal was done through the exchange in a series of transactions performed by SCB Securities, UBS, KGI Securities, Trinity Securities, Phatra Securities and Thanachart Securities, Kittiratt said.

"We have not heard anything about this matter as of yet," said KGI Securities spokeswoman Saengdao Puegpolpol.

Officials at other brokerage firms declined to comment or were unavailable.

In the weeks before the deal was announced, Shin repeatedly insisted it had no information from its major shareholders. Critics say the exchange should have pushed the firm to comply with disclosure laws. The role of the Securities and Exchange Commission has also come under scrutiny.

"We are questioning why the SEC gave many benefits to the deal, especially on tender offer issues. This is not fair to small shareholders," said opposition Democrat MP Korn Chatikavanij, a former head of investment firm JPMorgan in Thailand.

Under the deal, the buyers are obliged to make a tender offer for all remaining Shin shares. A similar offer for Shin's 43 percent-owned unit, Advanced Info Service, Thailand's largest mobile phone firm, is also planned.

But investors reacted negatively to the AIS tender offer priced at 72.31 baht per share, roughly 30 percent below the trading price before the deal was announced.

AIS fell 4.81 percent to 99 baht Tuesday, after earlier dropping to as much as 96 baht, which was its lowest since July 12.

The Temasek group received a waiver from making tender offers to shareholders of units Shin Satellite, broadcaster ITV and Internet provider CS Loxinfo.

"The buyers don't need to make tender offers for all Shin units, and the offer price for AIS is below the market price. Such a deal only benefits the Shinawatra family," Korn said. REUTERS