Nearly 30 subsidiaries of South Korea's family-controlled conglomerates may be newly subject to a ban on inter-affiliate trading when the nation's antitrust watchdog toughens a related rule, a market tracker said Wednesday.

In a policy report last month, the Fair Trade Commission said it will seek to revise the law forbidding inter-affiliate trading within a business group whose owner and family hold 30 percent or more of listed affiliates by lowering the threshold to 20 percent. For unlisted subsidiaries, the limit is 20 percent.

Should the law be amended as reported, 28 more subsidiaries of the 57 conglomerates with assets of 5 trillion won or more would be subject to the ban on inter-affiliate trading, according to CEO Score, a website that monitors conglomerates.