The securities watchdog uncovered evidence that Paiboon had made Matichon share transactions via the trading account of Patcharee Manoonpanich and others between June and September 2005 in such a way as to take unfair advantage for personal benefit, the SEC said in a statement.

In September 2005, Paiboon announced that he and a subsidiary of his GMM Grammy Group would take control of Post Publishing and Matichon through takeover bids worth a combined Bt2.67 billion. Matichon's share price almost doubled in September 2005 from Bt8 at the beginning of June 2005.

However, Paiboon eventually decided to drop his acquisition bid after a massive public outcry and sold a 12-per-cent stake back to Matichon founder Khanchai Boonpan for Bt11.10 a share - the price he had paid for them.

The committee also imposed a fine of Bt333,333 on Patcharee for acting as Paiboon's accomplice, the statement said.

Commenting on the SEC's decision, Paiboon said, "This mishap was the result of misunderstandings and a lack of experience among people at the operating level, and I can't deny responsibility for that. However, I must insist that all through my life, I have never considered benefiting from something like that and never before incurred this kind of tarnished record."