The Stock Exchange of Thailand (SET) warns that price manipulation and insider trading are on the rise due to booming share prices in recent months.
Supakit Jirapraditkul, head of the SET's market supervision division, said market officials have traced "many cases" related to price manipulation and insider trading over the past three months.
Information on irregular shifts in prices or trading volume have been tracked and forwarded to the Securities and Exchange Commission for further investigation and possible criminal charges.
Mr Supakit said regulators have tightened rules to guard against speculation, with the number of stocks now requiring 100% cash for trades rising sharply in recent weeks.
Stocks placed on the SET's cash balance list face a one-week restriction from margin lending trades, and investors have to settle solely in cash up front.
Since the start of the year, the SET has listed 28 stocks on its one-week cash balance restriction list compared with 48 stocks all of last year and 30 in 2011.
The cash balance list is selected based on the SEC's weekly turnover list, a list of stocks announced by securities regulators each Friday as potentially speculative based on turnover ratios and the difference between current market prices and stock fundamentals.
"The aim of the measure is to caution investors that these share prices may be trading outside their fundamental value," said Mr Supakit.
The SET also issues trading alerts to warn investors about unusual price movements and may at times require listed firms to issue public clarifications.