The exchange board yesterday amended a regulation that allows for delisting if companies fail to rectify the situation after four years.

The new regulation, which is subject to Securities and Exchange Commission approval, requires a minimum of 15 per cent of listed-company shares to be in the hands of minority shareholders.

SET corporate compliance and governance executive vice president Suthichai Chitvanich said in a statement the exchange would name companies failing to meet free-float rules one year after they fall below the threshold.

If they continue to remain below the required level in the following year, the SET will warn them of possible delisting, a non-compliance notice will be posted and trading will be suspended pending a report.

They will then be transferred to the non-performing group and suspended until the matter is resolved.

The exchange will give these companies one more year to meet the criteria before delisting.

After the new regulation is implemented, the call market - introduced to penalise companies with low free floats - will be revoked.

Securities listed on the call market are allowed to trade for only 30 minutes before the formal market opens at 10am, and for 30 minutes after the formal close at 4.30pm.